Although this is really funny it should only be used as a last resort IMO.
Doing these kind of semi-blackmailing practices can further harm the relationship with your client and it's probably not even legal in most countries.
Tim Van Dijck It's also not legal not being paid according to a contract, but here we are. This is, as I see it, a way to subtly f**k with the client if it's a small project where you don't really care if you'd get paid and you don't want to deal with the court, lawyers and so on.