Continuity mistake: When Tony and Pepper hug after reuniting on Earth, Pepper's hair is resting against her back. The camera changes angles and there is suddenly a huge lock of hair going over her right shoulder and down the back of it. (00:09:24)
Continuity mistake: The length of Thor's beard changes multiple times throughout the movie, and not consistent with the passage of time. For example, when talking to his mother, the beard is ~4" long, but soon after, when talking at the lake with the others about Nat's death, his beard is 7-8" long.
Continuity mistake: Before Wanda hits Thanos she has dirt on her neck then it's clean.
Continuity mistake: Just after Thanos knocks the Iron Gauntlet out of T'Challa's hand, he attempts to run over to it, but Wanda lands in front of him. T'Challa is standing in the background, a couple of meters behind her when she first lands. It then cuts to Thanos, but when it cuts to a shot panning in on Wanda, T'Challa has suddenly disappeared from the background. (02:23:10)
Continuity mistake: When Nat and Barton meet Red Skull on Vormir, Barton takes out his sword and points it at him. As Red Skull is saying their names, Barton's sword rotates itself by 90° in between shots. Note the position of the slits along the bottom and the pointed edge of the sword. (01:48:55)
Continuity mistake: When Steve is telling Sam what happened after returning the stones, Sam asks how'd that work out for him and Steve says it was beautiful. Steve is looking directly forwards at the lake when he says this. When the shot cuts, he is suddenly looking at Sam. (02:46:20)
Continuity mistake: Just after Tony hugs his father, he says "Thank you for everything", before saying "you've done for this country" whilst turning his head away from Howard. The shot then cuts to Howard Stark, where Tony's head is suddenly facing more towards him. (01:46:15)
Continuity mistake: When Tony hugs his father, he places his chin down on Howard's shoulder. It cuts to a shot of Howard Stark, where Tony does not move his head. When the shot cuts back to Tony, his head is suddenly slightly lifted off Howard's shoulder. (01:46:10)
Continuity mistake: Just after Tony returns to Earth, Pepper approaches Tony and starts to hug him. In the shot focusing on Tony, his chin is shown pressed against her shoulder, and then he lightly closes his eyes. When it cuts to a shot with Steve in the frame, Tony's eyes are more clenched shut and his face is more pressed against her shoulder. (00:09:24)
Continuity mistake: When Rocket, Hulk, and Rhodey are trapped underground, a water pipe bursts, and they nearly drown. There is a shot of the three of them with only a few inches of air left. Ant-Man comes and pulls them out, but when they reappear on the battlefield, they're dry. There was at least a few minutes between when we last saw them and when they reappear on the battlefield because Rhodey has a new suit. However, it was not enough time for them to dry completely. Hulk's hair is a little damp, but Rocket's fur is bone dry and fluffed up.
Answer: Well since he wouldn't know the exact moment she sacrificed herself, he might have shown up before then and then just had to wait for everything to play itself out before returning the stone.
Phaneron ★