In general, the originator of a thread has quite a bit more leeway with his/her own thread than others.
Basically, it's okay for the thread creator to hijack his/her own thread with another question, but it's rude for someone else to hijack a thread in this way. Edit: Such a question is also much less likely to get attention. I think a lot of people ask questions in established threads, thinking that their question will get more answers, but this usually isn't the case (and if it is, then the original question will get lost, and the thread will become confusing). Most of the time, a question gets the most answers when it has its own thread.
However, threads can, indeed, evolve. This most often happens with the off-topic threads (take a look at the Nascar thread. FLCommunityFan and I had a discussion about video
games in it), but it can also happen in other threads, especially if the original question was actually just the tip of the iceberg.
For example, someone with an uncycled tank is having
ICH problems. We'll walk them through the ich problems, but we'll also work on the cycling procedure, because that is likely the source of the ich issues.
Of course, the forum is huge, and stuff gets missed, or is close enough to the original topic that it is allowed to slide.