Receiving /public_html/undefined error messages 1/10,000 page loads.

[clarkstoncracke]

I'm having complaints of users receiving a 404 and ending up here:
http://mywebsite.com/undefined



Looking over my error log from yesterday, I had 43 instances of this happening, and 429,000 total page loads. So it is happening few and far between. None the less, I'd like to correct it.

Does anybody have any ideas on where I should start to look? It is occurring when users click from a forum to a thread, and from a thread to a forum. Sometimes its the same thread, sometimes its multiple threads in a giving day.

What normally causes a user to simply be forwarded to a page that is called "undefined"

Thank You