Many members of my community have complained about the following scenario:
Go to a forum you want to search.
Enter a search query top right.
Hit enter / "Search".
You are not completely happy with the results and enter something different in the search box top right.
Suddenly the search is site-wide.
People expected the search to still work in the scope of the forum you wanted to search originally and not suddenly across the entire site. I have been telling them to use the tiny button called "Search Again" to modify their query, but I think this is a usability issue as many people have reported it as a bug and thought it was bad.