Garry Marshall admits that when he envisioned Fonzie he was thinking of "someone who is cut" like Sylvester Stallone or Perry King, not physically someone like Henry Winkler, i.e. short. But he said Winkler nailed it in the audition, he had the attitude of Fonzie down, if not the look, so he got the part. Ironically Winkler admits he was channeling Sylvester Stallone when he auditioned for Fonzie. (He met Stallone when they worked on Lords of Flatbush together .)
Some of the extras in seasons 4 on are wearing the modern Blue and and Gold cap of the Milwaukee Brewers in stead of the Milwaukee Braves. The Brewers didn't exist during the time setting of the series.
The syndicated version called "Happy Days Again" used an alternate version of Bill Haley's "Rock Around the Clock" theme song. The original series used a specially-recorded version, but "Happy Days Again" used Haley's original 1954 recording.