I believe officially the conductor can tell you that you must head to the cafe car, assuming that the train actually has one, and sit there such that someone with an actual reserved ticket can have the "real" seat. But beyond that I don't believe that they will force you to stay behind and wait for the next train, unless maybe the standing room is already gone.

Besides, in most cases the conductor won't know what kind of ticket you're using anyhow, until after he tries to collect it, which is generally after the train has already left the station and of course that makes it rather hard for the conductor to deny boarding.