Random 404 ERRORS that do not go away until browser cache cleared

[phatmass]

Many users are complaining about getting 404 errors in my forum when accessing URLs that should be working. One of the things that causes it for some people is trying to post a smiley in a reply. They will get a 404 error and then the 404 error continues on the entire forum until they empty their browser's cache.

This happens to me occasionally as well. Once the 404 error occurs, it keeps occurring until my browser cache is cleared. BUT, I can access the forum if I type in the "index.php".

For example, this results in 404 error: site.com/forum/
But this does not: site.com/forum/index.php

Any clue as to what is causing this? Anybody else getting this? It's frustrating because user's who do not know about this bug just assume the forum is down and don't bother coming back.