You would think that Valve would have fixed it by now :/ Never mind that.
Valve claims to have fixed it on 17/12/2010 however people are reporting, in this topic, that the bug still occurs.
Players simply fall through the elevator on No Mercy map 4 if any physic prop goes through them.
This plugin will check the survivors Z position compared to the elevator. If lower, the plugin will teleport them up to the elevators Z position.
Description:
Provides a work around for the current bug with players falling through the elevator on No Mercy
Threaded Mode