I have, in my current campaign, a player with a compulsive liar character. The character also has a very low(negative bonus) wisdom score. He "believes" he's a polymorphed red dragon, but forgot how to return to form. The character prays to Tiamat sometimes, and sometimes I do stuff with it(flam spurt from the ground 1d4)sometimes not. Once he told other PC's that an empty room he'd explored had invisible potions in it... Needless to say, I put invisible potions there when the others went to look.
There's other stuff... but it's too late to write more...
later