If you have a large item tag (e.g. with multi line lore and enchantments) and it is hovered over in your inventory in a villager GUI, the villager's items for trade will be layered on top of the item tag.
Reproduction:
1: Get an item with a very large tag, for example:

/give @p diamond_sword 1 0 {display:
{Lore:["This","Lore","is","VERY","long","to","help","fix","bugs","and","with","this","lore","We","can","fix","bugs","because","of","this","very","long","lore","that","is",":?:","lines","long","so","we","can","fix"]}}

2: Go up to a villager and right click on him to open his GUI.
3: Hover over that large-tagged item in your inventory, once you're in the villager's GUI.
4: Voilà! If it goes up into where he requests his items, the item is layered in front of the text.

Screenshot: http://i.imgur.com/lo5FouI.png