Special Nostalgia Quiz Episode

This would only work for quiz shows which have run for at least a few years, depending on the length of each series. But after a certain number of episodes (say 70 or 80) have a special episode which consists entirely of questions previously asked on the show. Unless someone had an incredible memory and watched every episode they would only have a moderate advantage (if this advantage turns out to be too significant in practise, just half the prizes for this special edition), and you could gain an extra edge to the questions by comparing the performance of contestants to the originals (for extra fun, only ask questions which were answered correctly the first time around). For shows which have run for more than 10 years or so, a brief special series could probably be managed.