In PEACEFUL mode.
This has happened to me many times...
What I expected to happen was...:
After Initiating trade with a Villager, I was looking through his available trades. I expected to conduct a normal trade.
What actually happened was...:
But at that point, the program crashed.
Steps to Reproduce:
1. I suggest starting up a SUBSTANTIAL NPC Village. A villager population of 200-500 should do nicely.
2. To reproduce MY bug here, you should set your mode to PEACEFUL.
3. Initiate MANY trades among the villager until ALL of them have 4-8 available trades each.
4. Transfer from 1.6.4 to 1.7.2 such that some of the trades do not require emeralds in the trades.
5. Continue to trade with them until something crashes.