Happy Days - Joanie's Time Warp
In the episode, "Not With My Sister You Don't", Joanie's established age is 12. But, in the episode, "Passages", Chachi remembers the first time sitting on the couch with Joanie when they were 12 years old. (She met Chachi quite a few years later) Age inconsistency.
watch episodes, "Passages" and ''Not With My Sister You Don't"