The Other Slide Of Darkness - S3-E21
Plot hole: Quinn once again meets the alt-Quinn that he met in the pilot episode. Alt-Quinn tells our Quinn that he remembers solving the final part of the equation on his chalkboard. The problem is, there are many...possibly an infinite number of Quinn doubles in the multiverse. How could alt-Quinn have possibly known he was talking to our Quinn?
Chosen answer: Quinn's brother was going to be a recurring character, so "introduced" in this case referred to the actor becoming a semi-regular.
Jean G