These are a form of self-refutations, like "There is no truth," or "This sentence is not in English." Ironically, religious dogma most often DOES involve "should" statements.

Here are a few of my fave self-refutations - see if you can figure out why:

"It's true for you but not for me."

"No one can know any truth about religion."

"You can’t know anything for sure."

"Only fence-sitters are the truly enlightened ones."

"Only science can give us truth."

"You shouldn’t judge."

"You shouldn’t force your morality on people."

And some funny ones:

"No hablo Espanol." or "Je ne parle pas francais."

I will bet we can come up with some really great self-refuting statements made by pro-aborts, eh, Pere? Or at least terribly hypocritical ones like "The zygote is just a clump of cells!"