In an update that is long overdue, Instagram will now automatically correct an Instagram username to a Twitter username when the photo is shared in Twitter.
In the past, Instagram usernames were simply included in the Tweet. For many users, their Instagram @ name and their Twitter @ name are different, meaning that mentions in an Instagram photo shared on Twitter would not be linked to their Twitter account. Best case senario, the mention would present as an error; worst case, it would link to a completely different account.
If an Instagram user does not use Twitter, their @ mention in the Tweet will simply be shown without the @ symbol.