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Are you a

Toaster?

I'm famously rectangular

Most toasters are basically rectangular, as is most toast

Sometimes it's dark
Sometimes it's not

Bread can be toasted a lot or a little

If you use me while I'm wet
You might get killed!

A toaster in the bathtub can be a shocking experience

Are you

a flag ?

I'm famously rectangular

flags are famous (country or other localities flags) and rectangluar (except a few, e.g. Nepal)

Sometimes it's dark

e.g. pirate flag

Sometimes it's not

e.g. white flag

If you use me while I'm wet, you might get killed!

WETFLAG is an acronym for steps to take for first aid for children in medical emergency, see e.g. here : Weight Electricity Tube Fluid Lorazepam Adrenaline Glucose

Lightswitch?

I'm famously rectangular

Lightswitch plates are almost ALWAYS rectangular, as is the protruding piece you flick.

Sometimes it's dark

Light off

Sometimes it's not

Light on

If you use me while I'm wet

This has happened to me

You might get killed!

Electricty across the heart can be a real bummer, eh

Are you a

Handkerchief

I'm famously rectangular

It's also known as a pocket square

Sometimes it's dark

Different colors or designs, or perhaps blood

Sometimes it's not

Lighter colors, OR could be a wordplay— "sometimes it's snot"

If you use me while I'm wet You might get killed!

Wet meaning already used by someone else. Especially in older times, such poor hygiene would spread disease and death.

Are you:

Colorado?

I'm famously rectangular.

Colorado the state is about as close to rectangular as a piece of the Earth's surface can get, and appears rectangular in several map projections.

Sometimes it's dark / Sometimes it's not.

COLOR ado? Or maybe, referring to appearance of the river in various places or at various times?

If you use me while I'm wet / You might get killed.

In many parts of the Colorado River nowadays, if there's more than a trickle then it's likely because of flash flooding.

Are you a

Display screen

I'm famously rectangular

Screens are nearly always rectangular, to the extent that the phrase "square eyes" is used to derogatorily refer to someone who spends too much time looking at one.

Sometimes it's dark

Screens are black when off.

Sometimes it's not

They're bright with light when they're in use.

If you use me while I'm wet

You might get killed

Like any electrical equipment, screens are typically dangerous when exposed to water.

I don't know that I really like this answer but are you a

road

I'm famously rectangular

Roads are long rectangles (until they bend)

Sometimes it's dark

Sometimes it's not

Outside when you are driving

If you use me while I'm wet

You might get killed!

This is what lead me down the path to this answer...

I was thinking of the slippery when wet signs you see on roads, at least in the US

Are you a

light switch

I'm famously rectangular

most are

Sometimes it's dark

when its off

Sometimes it's not

when its on

If you use me while I'm wet
You might get killed!

Well, some people think that

Are you:

a mobile phone?

Rectangular in shape, mostly black or dark colors, and can kill if used when wet!

I'm thinking you're:

Cruise control

I'm famously rectangular

Indeed. Almost every card driver knows it.

While not all are in a rectangular shape but some are:

Sometimes it's dark

Sometimes it's not

Depending on how your cruise control paint color. And the brightness inside your car.

If you use me while I'm wet

You might get killed!

If you use it on rain, well you risk of accident as stated here. Everyone know you risk your life when having an accident.

Are you

an electrical circuit with a light bulb and a switch?

I'm famously rectangular

Circuits drawn on paper or digital images (used for example in learning books) almost always have a rectangular shape.

Sometimes it's dark

Sometimes it's not

When the switch is off, a light bulb does not receive energy and is also off, so "it's dark" (this explains why there is "it's dark", not "I'm dark")

If you use me while I'm wet

You might get killed!

Of course, this is about a risk of electric shock.

I think you are

A phone

I am famously rectangular

Non rectangular phones are rare

Sometimes it's dark

The screen when the phone is turned off

Sometime it's not

The screen when the phone is turned on

If you use me while I'm wet, you might get killed

A phone contains electricity, so it can be dangerous

Are you a

Zebra crossing

I'm famously rectangular

The Beatles walked across one in Abbey Road and the lines are rectangular

Sometimes it's dark

The black lines

Sometimes it's not

The white lines

If you use me while I'm wet
You might get killed!

They can become slippery while wet and if you fall you might get hit by a car

Summed up in this image:

Here I go~

You are a

Dance floor!

I am famously rectangular

Dance floors are pretty rectangular, especially those with checkered light patterns you see in disco clubs.

Sometimes it's dark
Sometime it's not

Places with a dance floor usually have lights flickering on and off.

If you use me while I'm wet, you might get killed

Slipping on a wet dance floor while dancing might get yourself killed.

You're

Wall Socket

Famously Rectangular

Wall Socket are Rectagular

Sometimes it's dark
Sometimes it's not

Some Wall Socket have light on them. Also it can be reference that you
can plug a light into it.

I'm penetrable and deep

You can plug into wall socket.

I'm a joy for the kids

Kids like put their fingers into wall sockets.

But if you use me while I'm wet,
You might get killed!

water + electricity = dead

I think the riddle is too vague.

It could be a

Runway

or a

Bar of soap

Well a few people have guessed something similar to this, but not this exact object.

Are you

A tubelight ?

I'm famously rectangular

Tubelights are rectangular

Sometimes it's dark

Sometimes it's not

Tubleights are notorious for flickering when initially started (thus sometimes its dark sometimes its not) [though could just be a reference to the fact they are a light with a switch]

If you use me while I'm wet

You might get killed!

Using any electrical appliance when wet is just a bad idea

This is a bit of a guess as I don't know enough, but are you

A pokemon

I'm famously rectangular

As in a pokemon card

Sometimes it's dark

Sometimes it's not

I know some pokemon have 'dark' versions of their character

If you use me while I'm wet

You might get killed!

Here I'm not sure. Maybe there is some card that means it gets killed with water?

I actually have two guesses. First:

Parachute

I'm famously rectangular

Many parachutes are rectangular, particularly the steerable ones.

Sometimes it's dark

Sometimes it's not

Parachutes come in many colors

If you use me while I'm wet

You might get killed!

A wet parachute may not open properly and may be too heavy, causing you to fall to your death.

And second:

Blanket

I'm famously rectangular

Blankets are almost always rectangular

Sometimes it's dark

Sometimes it's not

Likewise, blankets come in many colors, but you've also got the point that they are generally used at night when it's dark, but also can be used in the daytime.

If you use me while I'm wet

You might get killed!

A wet blanket can cause you to die of hypothermia.

Not sure you are allowed to give two answers, but here are two:

You might be a

1) TV 2) Swimming pool

I'm famously rectangular

1) Indeed! 2) Mostly

Sometimes it's dark
Sometimes it's not

1) Off/on 2) night/day

If you use me while I'm wet
You might get killed!

1) As any other electric devices (e.g. toasters), if they get wet and you plug them in, you might die of electrocution. 2) If I'm full up, you might drown!

Too many similar answers. Here comes a weird one. Are you

A pool for spent nuclear fuel near let's say a nuclear reactor

I'm famously rectangular

Most pools are rectangular.

Sometimes it's dark

Nearby nuclear reactor not running, all lights out. Note it doesn't say I'm dark but it's dark.

Sometimes it's not

Reactor running, all lights on at least for security

If you use me while I'm wet

The pool only needs to be full of water when in use

You might get killed!

As so nicely stated in the last paragraph in this what if you'd die quite fast trying to swim in a pool for spent nuclear fuel while in use.

Here is my guess

Floor tile

Rectangular, all colors design, and slippery when wet

OR

Stairs

Since climbing involved

You are a

trampoline

I'm famously rectangular

There are many rectangular trampolines.

Sometimes it's dark

It is black material.

Sometimes it's not

It could be another color though.

I'm penetrable and deep

They get deep when you jump down on them.

I'm a joy for the kids

Trampolines are fun.

But if you use me while I'm wet,
You might get killed!

It is dangerous to jump on a wet trampoline.

Sandbox

1. 
   

   Rectangular

   
   




2. 
   

   Fun for the kids

   
   




3. 
   

   If wet turns to quick sand

   
   

Are you:

Spongebob Squarepants?

I'm famously rectangular

He is a famous rectangle. All squares are rectangles.

Sometimes it's dark

Spongebob can have a dark sense of humor sometimes.

Sometimes it's not

The show can also be really lighthearted at times.

I'm penetrable and deep

He is filled with penetrable holes. And lives deep in the ocean.

I'm a joy for the kids

It's a kids show

But if you use me while I'm wet,

You might get killed!

If you pour water on your TV, you might get electrocuted

I really don't think this is the answer you're looking for, but I'm going to post it anyway.

You may be a

Roller Coaster

I'm famously rectangular

Roller Coaster cars are typically rectangular. This could also apply to multiple cars hooked together.

Sometimes it's dark

Some roller coasters have tunnels.

Sometimes it's not

Some roller coasters are out in broad daylight.

I'm penetrable and deep

The tracks can be "penetrated" through the holes between the rails. The tracks drop from a high point down into a deep valley.

I'm a joy for the kids

Lots of youth love roller coasters!

But if you use me while I'm wet,
You might get killed!

Heavy rain might cause some dangerous conditions for roller coasters. Also, when raining, there's potential for lighting strikes.

And per the tip in your comment, "There is climbing involved!"

Roller coasters climb up to the tops of some of the peaks.

Are you an:

Ipad or some similar device?