Calculator won't Quit? Help needed.

When using the Calculator application (the one in Applications / Utilities, not the Dashboard widget), it starts and works OK, but I can't exit the application without using Force Quit.

Is anyone else having this problem? Does anyone have a solution?

One possible issue for me might be that I tried the Safari security hole demonstration from 2-3 months ago, the one that showed the hole by launching Calculator. Perhaps that's part of the problem?