This is the first full-length feature in clay animation.
There's a longtime rumor that the "Mysterious Stranger" segment was cut from television broadcasts because it was considered likely to disturb children, but this was debunked by director Will Vinton in a 2013 interview with Animation World Network. "The internet is such a rumor mill of misinformation!" he laughed. "Nobody ever asked for it to be removed."
When the children meet the Mysterious Stranger, his world is a single island floating in a dark void. Mark Twain never finished The Mysterious Stranger, and only fragments of different versions he had considered exist.
The movie was completed in 1984, but wasn't widely distributed until 1986, to coincide with the passing of Halley's Comet.
When they are ejecting ballast from the airship to catch the comet, a Paige typesetter is thrown overboard, and Mark Twain comments, "Worst damn investment I ever made." In real life, Mark Twain invested, and lost, a lot of money on an automatic typesetter.