I have noticed that when I search and specify a forum, this selection is not preserved when my results come back. From the second search page (which shows results) I can re-select the forum and search again. From there it respects my forum selection.
Steps to reproduce:
1. Click on the "Search" link in a forum. 2. Enter a search keyword. 3. Select a forum to search (often rules for me). 4. Press Search. -. Note that you receive results from all forums. 5. In the search dialog above your search results re-select your chosen forum. 6. Press Search. -. Hey presto, now you see results for the forum you specified.