I know some servers which force the client rate to 35000 or 40000, even if it makes no sense at all since the rate is hard limited in the engine with 25000. This is a feature of ETPro were servers can automatically set a cvar on the client if the server requires it (and do a vid_restart if needed). The client usually doesn't notice this (maybe only in case of a vid_restart, where the map loads twice). And when the clients connect to a server with a better(?) setup then they get kicked for cvar violations. I guess most of you already encountered a player who was kicked for the rate cvar > 25000 or another weird setting (like r_uifullscreen).
My strong guess is now, that some servers exploit this by forcing clients to set well known cheat cvars - and when the client connects to a PB enabled server he gets kicked/banned as cheater. I didn't encounter such a server myself by now, but had some reports that players had cheat cvars set for no reason.
To prevent this you should:
Write protect your config
Play only on trustworthily servers you know