I don’t know about not true, but “Don’t look a gift horse in the mouth” is really bad advice. Horses ain’t cheap, nor is their dentistry.
@Kropotkin: that is the reason disgust is such a visceral feeling. The act of excreting feces is supposed to be pleasurable. The body makes this so to maximize its chance to remove harmful waste, and people report the scent of their own feces as somewhere between tolerable and even enjoyable. However, whether it’s because of a slightly different scent or simply the mere knowledge that it’s someone else’s feces, disgust exists to create a visceral, bodily rejection of other people’s feces. It’s vital that we get rid of our own waste, and pleasure helps us do that; it’s also vital that we avoid contact with other people’s waste, so disgust counteracts the arousal of sniffing someone else’s flatulence or fecal matter. Similarly, parents report the scent of their own infant’s feces isn’t as bad as that of other kids’ feces.
The second one @Hypocrisy_Central mentions is often mistaken as a Bible verse.