I'm willing to bet that many know they exist but they like to keep them quiet to exploit the bugs rather than to put them out in the public eye which may cause them to get closed.

I can think of a few on UA that are well known/discussed including using a SWU/CR-1 across multiple regions or in multiple directions as long as a stopover does not occur and you don't transit the same point, the ability to hold a reservation using a bad credit card, the ability to re-use eCertificates (now mostly closed).