After I had set up a station for testing villager careers, I found that the Career_tag was being changed from the one I had applied to it!
e.g. >> /summon Villager ~1 ~ ~ {Profession:0,Career:1}

I had some command_blocks on a clock testing that villager's career_tag. It read correctly until I opened the trade menu with it. When I looked at the output then, it read a different career!

It seemed that the career switching was random (no pattern I could see).

To reproduce the bug? Test a villager's career_tag after you summon it but before you trade with it. Then open the trade window with the villager and test it again.

EDIT

Alright... I've made some changes to my Villager Testing Lab and now it runs multiple testfors on a clock. Now checking for profession changes, CareerLevel changes, and the career of the villager in the cell.

I set the CareerLevel tag to 1. When I open the trade window of the villager, ++CareerLevel happens (it increments by one).