Leave it to American public transit operators to make things unnecessarily difficult.
Don't use your TAP cards in San Diego. According to the MTS (via MetroRiderLA), regardless of the fact that these secure smart cards do well and truly verify that you paid your fare, the fact that they have a different graphic on the front may expose you to a citation.
Metro! MTS! Make with the talky-talky so we can make these cards interchangeable.