EDIT: I SWEAR I WAS IN THE BUG REPORTS SECTION. Please move this, sorry! I'm also uploading a screenshot.
This didn't seem to be in the known bugs list so I hope it's not already been listed. The bug is basically as described, and I've been able to replicate it at will.
First, I play Prince and set a Noble Brigand aside. On each turn afterward, the Noble Brigand plays automatically as it should. On any turn I wish (the next one, two turns after, etc) I buy another Noble Brigand. This causes Noble Brigand to be played against my opponent (Lord Rattington).
I'm fairly positive that an on-buy effect is not intended behavior, and I don't know which other cards Prince might do this with.