Ever think that Brynjolf's infamous "Falmer Blood Elixir" was too good to be true?
Well... It's not. It actually does exist, but kinda svcks.
http://www.youtube.com/watch?v=N22W8teKAPs
Given the disparity between the effects that Brynjolf claimed it had and the actual effect I'd say his claims were too good to be true.
Still, I never noticed that Haelga had bought some - guess she isn't too bright, eh?