I have witnessed this too, though I can't place what's causing it. I don't think it has to do with server lag either, as I have experienced it with a normal 40 ping.
When picking up a weapon, there is sometimes a delay (or not even picking it up at all, and having to press the key again) I see this mostly with grenades since that's what I pickup the most. The client makes no indication that it failed to pick it up, quite simply nothing happens. I'm unsure of whether this would be apparent in a demo, though I will grab the demo if it happens to me.