Hard bug: if you have two blocks adjacent to two blocks of an enemy, it will put X's over your blocks and not allow you to move. This has happened twice with my core and gun blocks, forcing you to reload the page.
Okay, I've just tried to reproduce this and it's not crashing. Not in the browser or on my machine. And I can still move afterwards.
I'm probably misreading your directions. If you remember any more details or can reproduce it by opening the editor (press P) that would help a lot. I'll keep an eye out for anything like it for now.