I heard this today and it boggles my mind:
The proof of the pudding is in the eating.
Edit: So my friend Cindy pointed out that the incorrect saying is "The proof is in the pudding." - this is actually a legitimate saying! So I have to retract this one, since it is not a mixed metaphor.
But I will just add this: The proof of the pudding is eating a horse.
(This isn't too far off, since gelatin is made from hooves and bones.)