I've had that happen, but only if someone's worked a 26.0 bar into a 25.4 clamp (not that uncommon a situation), or if it's simply been oxidized onto place; in which case soaking it in household ammonia for a day or two does help to break down some of the oxidation. That, and a lot of Liquid Wrench.

The 'penny' method is the one I've used (and it's usually worked), but If you try the 'sideways screwdriver' method, be very careful in applying torque and rotating the driver blade: cast aluminum can snap.