But then, to be devil's advocate, if we are complaining about metagaming, wouldn't this be a situation in which the DM can metagame? The DM sees the die roll and then purposefully sets the DC higher or lower. Really, the DM should probably just say its not possible or not impossible, but some DMs don't like to own their decisions.
I guess a lot of it to me boils down to "if you can't trust the people you are rolling dice with (on either side) then stop rolling dice with them."