Thanks for the reply. Yes, PB has been working fine under VirtualBox. The anomaly was definitely caused by some kind of interface problem with my machine -- either the keyboard or the mouse. It's permanently resolved now.
I'm sorry this thread appears to indicate, on the face of it, that the compiled PB code was inconsistent in its behaviour. I'd be happier if this thread is deleted to be honest, as it might give new users the impression that PB had a fault, when it clearly didn't.