If a villager has a custom trade set to give you a map trading to get the map will result in a client crash.
STEPS TO RECREATE:
1.Create a villager with a custom trade (I recommend mcedit and the sethbling shop filter
2.Go into mine craft and try to trade with that villager
3.your client will crash.
EDIT: After playing around with the bug some it appears that if you /give maps item id the map to yourself it trades properly.

EDIT2: I uploaded a video clip of the bug http://db.tt/LcE9REA6 i have also added the crash log.