In the flashback showing Fez's first day at school, Hyde asks him what his name is. As Fez is saying his name, the school bell rings the entire time and Hyde says, "Yeah, I'm never going to remember that". Wilmer Valderrama (Fez) was actually stating the first names of all the actors on the show, with "Ashton Kutcher" as the first name he states.
The Todd Griffin version of "In the Street", played in the final scene, was the show's theme song during the first season. It was replaced by Cheap Trick's cover of the same song at the beginning of season two.
The blackboard reads, "CLASS PICTURE DAY", but later, Kelso erases the "CL" so that it reads "ASS PICTURE DAY".
Easton Gage, the boy who played Hyde at 7, also played a younger version of a Masterson brother in "Malcolm in the Middle".
This episode is season 4's 20th episode, making it 4:20. Likely why they made this almost a celebratory occasion for the series, with the end featuring stills of the past four seasons to commemorate the special number and it's link to the series' subject matter.