I have noticed this issue a while now, but just assumed that it had been reported already (since it is easily produced). However, didn't see it anywhere in the forum just now, so I thought I'd post it.
See attached screenshot for illustration: the antidote in the third item slot shows an additional 9, though it is just a single potion. This happens when I switch from Imoen, who has a Wand of Magic Missile with 9 charges in the same slot, to Jaheira. The 9 for some reason just sticks around. It doesn't happen when I switch to someone with an empty third slot first, nor does it happen to the Wand if I switch back to Imoen.