If you use OnPlayerRunCmd to block certain actions, like attacking or jumping, the client will experience the start of the action before the server can tell them to stop.
This is because Valve made our clients "predict" what we want to do. For example when our client sees +attack it starts the weapon fire animations, plays the gunshot sound and then resets. While the only one seeing this effect is the client, it can freak the client out and probably cause them to complain.
I found a server cvar that actually forces the client to turn prediction off. The cvar is sv_client_predict. Any client that joins after setting this cvar to 0 (-1 to disable it) will have prediction turned off. Unfortunately this forces us to disable an important feature to reduce the lagged feeling.
cl_predict is the client cvar, unfortunately it is flagged as a cheat. I haven't experimented with it much, but maybe we can use this to toggle prediction per-client.