Example line out of whois:
Kevin_b_er on @#test. #otherchan
The actual channel I'm in is #test. (with period)
but if I were to right-click that, and try to 'join channel', 'list users', or 'show topic', the commands executed would be on #test (w/o period) and not on #test. (w/ period)

Evident on mIRC 6.12 and 6.14, tested with and without ops. Tested with no channel after the period-ending channel in the list. Tested in channel and status window. Same effect in all test cases as outlined above.

This is probably to allow people to say "I'm in #blah.", and still be clickable, but now they conflict in something when . is a valid char for a channel name.