Thanks for the help, I think I may have figured it out... do you have the "Tab key changes editbox focus" option enabled in the Options/Other/Keys dialog? If I enable this option I can reproduce the problem.
If this option is enabled and the Tab key is pressed in an empty editbox (or in a situation where tab completion is not necessary), mIRC changes the focus to the next editbox/listbox. Triggering the the TABCOMP event in this situation did not seem appropriate.
I will add a note to the on TABCOMP section in the help file describing this situation.
