heh, I can guarantee that when I say 'click', I'm refering to a single depression of the left mouse button in no rapid succession to its previous use.
I did discover something new, I think. It only seems to occur if you have a horizontal switchbar with multiple rows, and you are switching between windows between the two rows.
Try to reproduce this bug for yourself.
Type: /JOIN #A,#B,#C,#D,#E,#F,#G,#H,#I,#J,#K
If you have a really large screen res, normalize mIRC so you can resize it smaller so channels span multiple lines.
I can't seem to come up with any consistant pattern, but just click around buttons for long enough and you'll see it happen. Make sure that you click on the same button twice (slowly) though, which should have the effect of making the previously active window active again. It's after doing this a few times that it gets spastic and flips around with a mind of its own.