R&R, please explain why the Old Catholics were wrong. This should be interesting.
Old Catholics rejected the definition of papal infallibility because they decided it was contrary to Tradition. Before you rely upon the notion that a Pope is infallible when solemnly defining a dogma, recall that papal infallibility was the dogma in question and thus it's a circular argument. Pope was infallible in defining papal infallibility because papal infallibility is a dogma. So you can't use that.
Please explain why the Old Catholics were wrong to do what they did.
Heck, the Old Catholic "Declaration of Utrecht" opens with the exact same appeal to St. Vincent of Lerins that Traditional Catholics use.