you are a real lurker, aren't you?
I pop on now and again when Chris or Doug point out an issue. My "day job" keeps me too busy otherwise.
Most of the affected threads are older and I'm going to assume that the UBBCode didn't port well between version updates, or was altered slightly when we ran the "rebuild posts" function that is supposed to actually clean those types of things up.
Some posts can be corrected by us going through and specifying that they contain both types of code. Others we'll have to give more individualized TLC. In the end, I'm not so sure it's worth it unless it's truly bothersome.