I used to be in a School of Rock (if you don't know what this is look it up, not the movie either, its an actual club sorta thing for highschoolers) cover group for Iron Maiden. One of the bass players wrote some of the riffs in sharpie on their arm, I was like, "seriously, we had almost two months to learn a small handful of songs and you need that". I did applaud her creativity however.
When I was 21 and eagerly learning guitar I did a band workshop at Blue Bar school of music out on Fort Mason in SF. One of the tunes was Better Be Home Soon by Crowded House (singer changed the key to suit her voice, song lost all it's power). I don't know, 10 weeks of weekly rehearsals and I just couldn't remember that damn instrumental section. So on the day of the show I typed up the chord sequence and taped it to the neck of my guitar around the 12th fret. Problem solved!
Used to have the show on video, now since lost. I always used to chuckle watching that part of the tape. Still have the video of the 2nd one I did 4 months later though!