For everyone getting this error on some servers after the TFC update, go to Options > Video and turn off "Enable HD models if available" to be able to play there again.

Setting mp_consistency to 0 on the servers "should" also fix this.

More info on the TFC update here:
http://www.tfcommunity.com/forums/topic/549-team-fortress-classic-update-released/
http://forums.steampowered.com/forums/showthread.php?t=3168535
http://steamcommunity.com/games/20/announcements/detail/2125982923273800553