This bug only seems to affect trades which require an item and some emeralds in return for an enchanted version of the said item. Initially, the villager will demand emeralds as usual, but eventually (I think after using another trade with the same villager) it will stop demanding emeralds and will just enchant your items for free.

This is happening in a 13w36b server. It happened with all 3 villagers which were enchanting items in return for some emeralds, so it shouldn't be too hard to reproduce.