The case
This is one of the reverse cases: the detail is real and people are sure it isn't. Shaggy's lanky neck has carried a visible Adam's apple since the first Scooby-Doo, Where Are You! episodes in 1969.
It's a small line in a simple design, and simple designs get simplified further in memory. When people see it again as adults it reads as a change, which is exactly how these cases start.
Why our brains do this
Minor details in a simple cartoon get dropped from memory, so seeing them again feels like a revision.
Sources: Wikipedia — Shaggy Rogers · Wikipedia — False memory: the Mandela effect
