<<Just "forget" to put it on the medical questionaire.>>
Completely and utterly stupid and highly irresponsible, as has been said before on PPrune.
The medical is there for a good reason - to preserve certain standards of fitness. They medics are not idiots and will do all they can to pass someone; if they fail then there is a good reason and specialists will have been consulted.
Next time you, or your wife and kids, are flying in bad weather just wonder if the bloke up front, or the radar controller directing him might have some "hidden" medical problem which they had kept quiet about...