Whenever you change a villagers trade to air using NBT tag editing and open the trade the game crashes even if you don't trade. I used a mod to create the trade but the game will crash even without it on single player and multiplayer.

Steps to reproduce in 14w08a:

• Place command block with

  summon Villager ~ ~-2 ~ {PersistenceRequired: 1,CustomName:Lever Game Champion,CustomNameVisible:1, Attributes:[{Name:generic.movementSpeed,Base:0}],Offers:{Recipes:[ {buy:{id:0},sell:{id:323,Count:1,tag:{display:{Name:”Local Lever Game Champion”,Lore:[“Some day I will beat, the legendry Teep!”]}}}}]}} 
  

• Pausing the game will yield an exception in the development console:

  [19:49:40 INFO]: Client> [19:49:40] [Server thread/INFO]: Saving chunks for level 'New World222'/Overworld
  [19:49:40 INFO]: Client> s: Saving entity NBT
  [19:49:40 INFO]: Client> 	at sk.e(SourceFile:1123)
  [19:49:40 INFO]: Client> 	at sk.d(SourceFile:1090)
  [19:49:40 INFO]: Client> 	at atc.a(SourceFile:210)
  [19:49:40 INFO]: Client> 	at atc.a(SourceFile:99)
  [19:49:40 INFO]: Client> 	at oa.b(SourceFile:161)
  [19:49:40 INFO]: Client> 	at oa.a(SourceFile:192)
  [19:49:40 INFO]: Client> 	at ob.a(SourceFile:663)
  [19:49:40 INFO]: Client> 	at net.minecraft.server.MinecraftServer.a(SourceFile:305)
  [19:49:40 INFO]: Client> 	at cby.v(SourceFile:124)
  [19:49:40 INFO]: Client> 	at net.minecraft.server.MinecraftServer.run(SourceFile:387)
  [19:49:40 INFO]: Client> 	at mr.run(SourceFile:657)
  [19:49:40 INFO]: Client> Caused by: java.lang.NullPointerException
  [19:49:40 INFO]: Client> 	at aik.j(SourceFile:114)
  [19:49:40 INFO]: Client> 	at ail.a(SourceFile:125)
  [19:49:40 INFO]: Client> 	at aai.b(SourceFile:206)
  [19:49:40 INFO]: Client> 	at sk.e(SourceFile:1111)
  [19:49:40 INFO]: Client> 	... 10 more
  [19:49:40 INFO]: Client> [19:49:40] [Server thread/INFO]: Saving chunks for level 'New World222'/Nether
  [19:49:40 INFO]: Client> [19:49:40] [Server thread/INFO]: Saving chunks for level 'New World222'/The End
  

Villagers apparently cannot buy air. Doesn't throw exceptions with

summon Villager ~ ~-2 ~ {PersistenceRequired: 1,CustomName:Lever Game Champion,CustomNameVisible:1, Attributes:[{Name:generic.movementSpeed,Base:0}],Offers:{Recipes:[ {buy:{id:1},sell:{id:323,Count:1,tag:{display:{Name:”Local Lever Game Champion”,Lore:[“Some day I will beat, the legendry Teep!”]}}}}]}} 

Difference:

buy:{id:0}

-->

buy:{id:1}