Thought I'd best share this info.

Earlier today I got the following "Error verifying Steam user id ticket" and couldn't join any servers. My first thought was that my Steam account had been stolen Anyhoo, after Googling I found that all I needed to do was delete the "ClientRegistry.blob" file. Then I restarted Steam which did a small update and then I simply had to log back in. Job done.

i used to get this error on other games havn't had it on tf2 yet though