Another: Comments in scripts containing global variable names of object-internal variables (like //global.image_xscale = 2;) could cause undefined and seemingly random behavior where trying to access the image_xscale variable in an object would sometimes succeed and sometimes do nothing. (Studio 1)

And another: Very large projects made in GM8 could get the program to delete its own file related to compiling upon running the game, and you could fix it by copypasting the file back in after every Run.

My arcane GM knowledge that I collected over the years finally has a use :^)

Here's another obscure tidbit:

Did you know that in old versions you could freeze the game on infinitely executing the step event by setting image_yscale to 0 on an object with a sprite that is only 1 pixel big?