Check out this thread:
http://www.threadsdev.com/forum/showflat.php/Cat/0/Number/101603

There's a query there to verify the indexes on your w3t_Last table are correct. In case there's a database problem on your site.

If it's just a few users, usually corrupt cookies or temporary internet files on their end. Also potentially caching issues on the browser end as well. This FAQ should help them:
https://www.ubbcentral.com/support/faqs.php?type=browser

Joshua Pettit
Web Developer
www.ThreadsDev.net | www.JoshuaPettit.com