United had agreed last week with the Championship Club on a change for the Welsh midfielder.
And the move became official on Wednesday, when James signed a five-year deal at Old Trafford.
But Manchester United did not let the deal go smoothly.
You have tagged the wrong Daniel James in an Instagram post as part of the announcement.
The original post wrote: "When @ DanielJames_97 smiles, we smile ALL!"
However, Daniel James is not the new United signer, although his follower account shot up after confusion.
The post was edited quickly and now contains the correct username of James – Daniel_James97.
But the fans had already noticed the mistake of the Premier League giants.
One had responded to the post: "Wrong tag bro!"
Another added: "wrong day"
A third wrote: "Change the username"
With a fan of United: "This is not the right Daniel."
Amazingly, United misunderstood the James day on Twitter AGAIN.
You marked @ Daniel_James_7 instead of @ Daniel_James_97.
James had an outstanding season in the championship with Swansea, in which he scored six goals and scored seven assists.
The 21-year-old is Ole Gunnar Solskjær's first new entry in the summer. The Norwegian wants to refresh his team.
And James can not wait to get started in Old Trafford.
He said, "For me it's really surreal.
"I think every kid is growing up and wants to play for Manchester United.
"I never thought I could play against them, no matter playing for them, it's surreal and I'm really looking forward to it, I can not wait to get started.
"I look forward to working with Ole, meeting the team and getting started."