Corvo is definitely a solid example; his magic is very limited/narrow in focus, and the stranger definitely fits the bill as a patron.

Quote Originally Posted by Particle_Man View Post
Are we talking about 5e or 3.5? The latter allows for ancestors to have made the pact.
You could do that in 5e too - the important point is that the pact is binding to the warlock, which could apply to careless wording by an ancestor.