In response to eric the red, the corollary of that one is usually

'And if you think that's rather high
Just go next door. The fuckers fly !'

One from my University days :

DO NOT WRITE ON THIS WALL

underneath in a different hand :

'You want maybe we should type ?'

And in a Gents, somewhere, sign on the wall read

'Please adjust your dress before leaving.'


to which some wag had added 'Wrong bog, mate'.