Continuity mistake: In the final battle, Wasp and Ant-Man are in the van trying to get the quantum tunnel operational. We cut back to the fight and we can see Ant-Man there too, fighting in his giant form. (02:22:20 - 02:23:00)
Continuity mistake: During the interview, while sitting, his striped pants are unzipped for a few cuts, then they are zipped in later shots.
Character mistake: Selena Gomez drives into town in a fully restored 1967 Pontiac Le Mans, which is a long box shaped car. However, when confronted with Bill Murray and Adam Driver, Adam Driver complements her on her 1968 Pontiac Le Mans, which is a completely redesigned shorter curved body style. The two look nothing alike.
Continuity mistake: After the speeder chase, everyone lands in quicksand. In one shot, 3PO is buried headfirst. In the next, he has flipped and his upper body is visible.
Continuity mistake: When Elton and John Reid are kissing in the cupboard John's hand keeps changing between shots. John's hand rolls down Elton's chest, then the next shot it's on Elton's cheek, the next shot he takes his hand away, then the next shot his hand is by Elton's mouth and Elton pretends to bite John's thumb.
Revealing mistake: In the beginning of the movie, when Hellboy is speaking on the phone with "Dad", he's speaking on on speaker, but when the phone shows up, instead of showing a screen of a call on speaker with the duration on the call, the screen shows what would appear if he was receiving a phone call before accepting or declining. (03:35:00)
Continuity mistake: When Anna's singing "The next right thing" she puts a bag on her shoulder, with the strap on top of her hair. After the camera spins round her, her hair is suddenly on top of the strap. This bears a strong similarity to the mistake in the original Frozen where Elsa's hair passes through her shoulder - might be a deliberate nod, or else animating hair around objects is difficult enough that shortcuts are often taken.
Factual error: The USA map is inaccurate for the year it is set in. It shows all states but in 1886, many of the northwest states (e.g. Montana, Idaho, the Dakotas) weren't states until 1889+.