Marvel used back-up stories both as a training ground for new talent and as a way to allow the creators of regular titles to catch up on their deadlines by having them write fewer pages, and I was the frequent beneficiary of both of those policies.
My first back-up appeared in Avengers Annual #6, and starred one of my favorite characters, the Vision. The eight-page story “Night Vision!” was drawn by Herb Trimpe, who was best known for his work on the Hulk.