Sometimes when I play this game on http://terrycavanaghgames.com/tinyheist, including on CoolMathGames, the whole game ends up blacking out in an unexpected way AND for no reason.
it happens to me too. I´m pretty sure it happens when I open too many locked doors too quickly such as on certain variations of floor 10 but on other floors I have no idea how it happens.
Edit: it also happens when I intentionally try to crash the game by having 20 patrol bots spawn and then getting caught by a security camera.