Going through my modest collection looking for errors and oddities (and freaks) I came across an interesting, singular error on the 1977-1982, 17cent, First-Class Definitive #789as.
A rare, double image:

How often these kinds of errors occur in the same printing, I don't know. But I hope and expect someone will comment on it to enlighten us all.
I must say it really looks quite striking!
Ugruk