Code can be unsafe. One of the easiest ways to screw up with code is what's called an infinite loop.
Apply the following expression to any property in order to crash After-Effects:

while(true){}

while() in JavaScript lets you execute a block of code repeatedly as long as the condition inside the parenthesis is true.
This means that it's your responsibility to break out of the loop. If we don't, like in the example above, our code will try to run forever and After-Effects will freeze as a result.
Another way to do that is with a for loop:

for (var i=0; i<Infinity; i++) {}

Using a for loop we can run a code repeatedly until a condition has been met. The condition we set is i < Infinity, which as you can imagine, will never be met.

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Sabotaging somebody's project file

Here in the Good Boy Ninja project, everything is about peace and love, but let's say there is someone you war and hate...
You could make After-Effects crash, but only when the timeline is set to a specific amount of time. Something like:

if (time > 10) {
	while(true){
	}
} else {
	value
}

Now every time they will scroll past the 10 seconds mark, their project will crash. Cruel, I know, but doable!