A few of my forum members have reported that when they visit, they see dates on the topics are several days old. But when they open a topic, there are newer posts.
For example, they might open a forum and see the last post is 12/31/2005 (3 replies). But when they open the topic, there are 10 replies as of 1/4/2006.
This seems to be a periodic problem. Is it a cookie problem? Would it help to have them delete their cookies?
A proxy server is a likely culprit. ISP's often use them to serve cached pages to their users and therefore reducing their bandwidth bills. If the users have the same ISP you could ask them to contact the ISP and have your i.p. address added to their no-cache list.