Bert and Ernie's apartment is clearly a sub-basement apartment in the 123 Sesame Street building, but often when looking out a window from inside the apartment, it looks as if it is on the first floor (i.e. the half-walls seen in front of the windows when outside the building aren't there, and when people are at the window, they're standing straight up, not crouched over as one would expect).
During the final stanza of the Anything Muppets' song "J Friends", when the four Muppets jump up at the line "Let's jump with Jane", the hair and forehead of Muppet performer Frank Oz are briefly visible at the bottom of the screen.
During the "Beat the Time" skit with The Count, Guy Smiley declared that The Count won when he created his thunder and lightning from counting the 20 seconds he had, as he had found two thing that came from the sky. However, the thunder and lightning came after the time ran out, so technically he still lost.