Odd user login error For some reason on my site, if a user logs in using the user login block it will send them to the /user/ page to login where they have to type in their details again and it will log them in. It will redirect them here whether the initial details are correct or not.

This is also happening with the slipjaq module (slide down login panel) so I think it might have something to do with clean URLs. Do you have any idea?
Try disabling Clean URLs and see if it works.
If it does, it might be a problem with the module you are using.

Do this behavior also happen if you disable slipjq module?
If no, I recommend that you build this functionality yourself using jQuery. It would be easy.
If yes:

Have you created any alternative URL's pointing or coming from /user ?
If yes, try removing it.