Plot hole: Zeus reassures the vast audience of deities in Omnipotent City that the God Butcherer won't reach Eternity, an omnipotent being that will grant one wish to the first person ever to reach it. This knowledge is fairly trivial amongst deities. This introduces the obvious absurdity that nobody ever tried to make use of this power to undo Thanos' damage. Also, Zeus just happens to know the bad guy's plan out of the blue.
Other mistake: In this movie Korg has two biological fathers (he calls them that way); his race appears to be formed by males that reproduce by holding hands in lava pits. However, the first time Thor met Korg in Thor: Ragnarok, his backstory was that he started a revolution that failed because just his "mum and her boyfriend" showed up.
Continuity mistake: After Thor asks for Zeus' thunderbolt, Zeus denies this and attaches the thunderbolt to the frame behind him. The next time we see Zeus in front of this frame the thunderbolt is gone, but in the shot afterwards when Zeus wants to kill Thor with the thunderbolt it's back on its place at the frame.
Continuity mistake: When Zeus "flick - flicks" the clothes back on Thor, the Zeusettes are watching in the background; the one to the far left is expressionless and with her hands down on the railing like the other three. In their close-up though, she is smiling wide and her hands are raised, only to make a pouty face and deflate a moment later.
Continuity mistake: The Viking tour ship they take to the shadow realm is called Aegir and it's written on the front side of the ship on the right-hand side where they get on board. However, when they get off the ship, they exit from the other side, and the name is either written backward or the shot is mirrored.