Towards the end of Act 1, before the fight with Alexander, if you opt to take Kerban with you and teleport your main character before the fight starts, Kerban will view this as a hostile act and attack the party.

I don't think it's a bug, but ... yeah, it'd be super if he could just relax and stay consistent with the rest of the game's internal logic, which seems to be "yeah, it's totally ok to teleport party members."