This has happened to me on occasion as well. From home, no less. I've always chalked it up to my connection since it is, at times, a bit fussy. As for the "keep me logged in" check box, the only one I'm seeing is a "Remember Me" box. No big deal for me, my posts are usually pre-written anyhow, but it can be frustrating. As a side note, however, I've seen this happen on a few other vBulliten Board driven forums as well.

GW