Problem with logging out

[#1]

When I logout from community, I get an error message telling me to log back in then logout again. This started about 3 weeks ago. Can anyone tell me  if this has happened to them and what is causing it?

[#2]

Doesn't happen every time but I've managed to reproduce this. Session verification failing after a period with the tab inactive. FF14.0.1 on Vista.
See attachment.

[#3]

I managed to replicate the Session verification failed. Please try logging back out and back in again by leaving my webpage inactive, this then times-out the session, more of a security feature than anything else.

[#4]

Thank you for your replies. I now understand why this is happening.