The title of this episode is a reference to "How I Spent My Summer Vacation". It's also a possible reference to Joe Strummer, frontman of The Clash, who would die roughly a month after this episode aired.
When Disco Stu confesses in the car, for a frame the white and purple on the left of his jacket switch places with each other.
When Elvis Costello takes Homer's guitar and drives back to get the pick, the guitar changes color from red to a mustard sort of color.
When the Rolling Stones are on the stage talking about the camp, they knew Otto's name even though they never met him before. This could simply be a reference to the popularity of the show's characters, including background and recurring.
After Homer and his gang get their instruments (all guitars)... and started singing, Homer's guitar changes form and color.
When the guitar crashes through Mick Jagger's window and messes up his desk, a stack of papers on the desk disappears for a frame.
When the audience of the final concert is shown, Lenny's hair is lighter brown than it is supposed to be in one scene.
When everyone is getting told that they can't all play guitar and are told that someone could play bass instead, Homer jumps over the counter and gives guitars to everyone, including the bass nobody wanted before.