Hard to explain, here's how to reproduce it:
1. Click Edit Post on any thread you have posted in.
2. Without changing anything, hit Edit.
3. You will recieve an error that the exact same text by you exists in the thread.
4. Without fixing the error, go to another thread in that section, scroll down and observe that the "error" has followed you to another thread. (If you go to another section of the forum, it won't follow you there, but it will be waiting for you when you come back.)
5. It will continue to follow you in that section until you change the text and hit edit, however, the edited text will now appear in whatever thread you happen to be looking at, and not in the original thread that generated the error.
Weird.