What is a rudder or fin stall?
Right side slip from left rudder, rudder stalls, what happens?

This is comparable to a wing stalling in that it is caused by the rudder/vertical stabilizer 
exceeding a critical angle of attack.  There for, it has passed its maximum deflection potential.  The result will be the fuselage slip will reduce until the rudder/vertical stabilizer assembly unstalls.  Just like with the wing and elevator, reducing rudder deflection will also unstall it.

Notice designers have learned 3 techniques to help prevent this:  1.  Lower aspect ratio vertical stabilizer. Just like wings, these stall at higher AOA.  2.  Swept or delta shaped leading edge.  Many 1940s era Vstabs were fantasticly ahead of their time had their planforms been made into wings.  These shapes will stall at higher AOA, allowing greater rudder deflection.  3. The all moving rudder.  Making the entire assembly a rudder (as on the SR-71 Blackbird) and making them large enough will help over come the directional stabilizing effect of the rear fuselage when turning.

Rudder stalling would most likely involve some fairly aggressive control application, which may exceed the structural strength of the vertical stabilizer.  Beyond fighters, there may not be much benefit in that type of manuever.