(All gamemodes are affected)

When playing I traded with villagers in creative and went back and fourth between villagers, when you open a different tab in a villager and then go to another villager, leaving the first villager on the second tab, then return to said villager, you can trade its second trade in the first tab.

Here is a video of the bug, because it is hard to explain...
http://youtu.be/W4NbtJcPFMo