This may have already been fixed in B5, but...
One hero found the Treads of Tuor, which give +4 group move. He then gave them to another hero, but is continuing to see the benefit even without them (+4 base group move). The other hero now has a -4 base group move, and while wearing the treads nets out to zero. Interestingly enough, the cost for him to improve his group move is now (-10). I'll be curious to see what happens if I try to improve it when (if) he levels up.