This also happens to me too and to many people I know. It turns out that Regen is a bit unstable and too sensible to code errors, like, if you try to access some prohibited memory areas, Regen may suddenly through an M68K debugger window that will continue popping up while 68K is doing something it's not supposed to do. It even may crash when running a bad code. Well, everything may happen, I even managed to crash Kega by passing a certain opcode, which crashes both Kega and Regen with no warning (I really should remember this code, it could be useful to have a close emulator option in my hack =P). But I don't recall Regen crashing when playing the game itself, if the game's code isn't crashy. It usually crashes when running a debugger or bad code. But, actually, I don't use Regen much, I prefer Kega for playing games and Regen for debugging my stuff and ensuring it'll likely work on hardware.