This page is outdated and probably misguided. Read as no more than suggestions, and use at your own risk.
A general guide to "What to do if something's not working" (they are in a general order, but it's not strict):
1) Check the actors that are causing the problem, as well as any relevant connections to them. If they're custom, check the code. Check all relevant settings. Sinking brushes has been known to cause errors occasionally if not done correctly. Make sure Events match Tags.
2) Rebuild. Re-check the problem; if it's still there, repeat step 1.
3) Check surroundings and ensure there's not an error (like BSP or collision) there. To do this, delete surrounding actors one by one until the problem disappears. DO NOT SAVE! Reload the map and delete or move the actor that was causing the problem.
4) Restart UEd.
5) Restart the computer.
6) Test the map on a different computer.
7) Check relevant files and ensure they're not corrupt or have a duplicated name (UT packages cannot have the same name, even if they have different extensions!).
8) Delete ini files in \UT2004\System (back them up first, but they should regenerate themselves the next time you run UT2004).
9) Check game settings.
10) Reset UEd preferences.
11) Copy the contents of a map into a new map and see if it works. If it doesn't but you think you know what could be wrong, check the concept by building an equivalent system in a new map.
12) Export the map to .t3d, open in notepad, and manually search for a problem.
13) Search for a similar problem on the BU (BeyondUnreal), Epic, and 3DBuzz mapping forums.
14) Search for a similar problem on UDN (Unreal Developers' Network) and the UnrealWiki.
15) Search for a similar problem on Google.
16) Ask a question on the BU, Epic, or 3DBuzz mapping forums.
17) If none of those work, start a new map or ignore the feature you're trying to create.