Continuity mistake: When America and Wong jump through the Star Portal just before Wanda destroys Mount Wundagore, you can see they will travel to Kamar-Taj. However, when America opens another Star Portal to retrieve Dr. Strange from the destroyed universe, they are standing back on the snowy mountain top.
Continuity mistake: At the wedding, Christine tells Stephen something quite hurtful; wide-eyed, he asks her how long did she have it in the barrel; a black partygoer walks behind him but is gone in the reverse shot. Moreover, she replies "Long time" and a couple walks past her, only to walk again past Stephen in the next shot.
Continuity mistake: When Christine and the groom have a look at the battle on the nearby building, in the shot from the front you can see to the left a very excited guy hop from foot to foot with his mouth open. In the angle with the camera behind Christine, the guy is directly next to her, but there were a couple more people between him and the bride.
Continuity mistake: When we first see Wanda with her kids (which turns out to be a dream, but it's also showing a glimpse from another universe, and it's not meant to be 'out of order or unnatural) she tries to tuck one in, and he refuses. Wanda pulls back, but in the next shot she yet has to.