The case
The extra-creme Oreo arrived in 1974 and has been a lunchbox trade currency ever since. The package spells the filling with one F, a deliberate trademark spelling like Kool-Aid or Froot Loops.
Because the cream is obviously "stuff," the eye reads the double F it expects. It's one of the few cases where people, shown the box, still insist it changed.
Why our brains do this
Real-word spelling overrides brand spelling, especially when the word is one you say every day.
Sources: Wikipedia — Oreo · Wikipedia — False memory: the Mandela effect
