Wow, I have never heard of this happening... Maybe your computer can't handle making maps. My friend had a problem similar to this and to this day, she can't figure out how to fix it, and neither can anyone else. I suggest searching through the help threads in the forum to see if there are other people with this problem. Best of luck fixing it. D:
EDIT: Or you could do what Rim said and save before testing. XD Unfortunately my friend tried that too and still can't fix her problem. >>