Seems to happen to me in every Source-engine multiplayer game. Any time I get a REALLY severe lag spike, as in my ISP isn't sending anything for around five to ten seconds, everything in the game gets quite screwy. About half the walls, buildings, vehicles, and such will disappear. The other half usually behave in highly abnormal manners. A prime example just happened to me this morning where I was lag-artying (a complicated process of hoping the enemy doesn't come to kill you). I jumped out of my NF arty to drop an ammo box for it, and... well... my connection disappeared for a bit. When I came to again, I was inside the NF CV model, for some bizarre reason. I could no longer weapon switch. Also, most of the nearby buildings/walls/turrets had disappeared. When I tried to re-enter my "arty" (which was now a CV model, apparently), the client crashed. Uhm, I'm thinking I'll send yonder dump into Krenzo, if it made one. This isn't really an important issue, seeing as how it seems to only affect an amazing one person out of the entire player base.
This actually isn't a 56k issue. There was a thread about this one before. I'll see if I can dig it up for you. You see this has happened to me too and I'm on a broadband and some other person had some chancing models in his game... Time to dig. Edit: Here it is: Something funky with CV models.