At some point in Tombs, after Selias joined my party in Kristophan, I discovered that my 6th character has turned into an NPC: in battles the computer took over its role, he couldn't pass equipment to other characters, etc.

Then, after Selias left and Tasslehoff Burrfoot joined my party in Hulderfolk Wood, then left it, his icon turned into that of a giant squid, at least in the overland encounters (before that his icon stayed the one I picked for him).

At that point I decided that something must be done about it, and remembered about GBC. It showed that, indeed, this character had non-zero value (179) of the NPC field. Its icon number though was the correct one (I suppose the picking of icons for the NPCs does not use the character's icon field). I edited the NPC field to 0 (with GBC), and the problem thus was solved (I hope).

Still, I decided that it could be useful to share this. It seems that the bug can happen when some NPC joins or leaves the party.