When you happen to erase the number in the box for the number of beats in the bar properties dialog (F5 or Command B), leave it empty and press "ok" the program will answer with an error message and gives you the option of pressing "ok" or "cancel" . No matter what you'll do than Band in a Box will crash.

I am using Band in a Box 2009.5 on Mac (Intel) 10.5.8

Good point. Will fix.