Trivia: In the beginning of the film, young Sonic gives Longclaw a flower with spiky petals. This flower is actually based on a flower sprite from the original "Sonic the Hedgehog" video-game for the Sega Genesis. (00:02:16)
Trivia: Thanos' gauntlet bears a striking resemblance to the relic of the hand of St. Teresa de Jesus, one of the most important holy relics in Spain.
Trivia: Stan Lee's cameo actually references another cameo he made as himself, in 1995's Mallrats - he's reading the script for that on the train. This marks the first time he's explicitly himself in a Marvel Cinematic Universe film (he identified himself as Stan Lee in Fantastic 4: Rise of the Silver Surfer), not just a random character. In fact Guardians of the Galaxy, Vol. 2 confirms he's actually playing the same character in every cameo, a Watcher Informant, roaming the multiverse and reporting back his discoveries. And given he's reading the Mallrats script here, that means Stan Lee himself was actually an alien being all along, observing humanity and others. And of course that means the real Stan Lee exists in the MCU, so created comics based on characters who really exist, albeit maybe in another universe...but you could go on like that forever.
Trivia: Samuel Jackson spotted that the eye patch his character wears in the movie switched eyes in the promotional posters announcing the movie in Los Angeles.
Trivia: When Olaf just starts to sing "When I Am Older" count for about 10 seconds and watch the fire jump from tree to tree. If you watch the right side of the screen closely at the 10 seconds point, you can see the fire briefly take the form of Mickey's head.