Daniel Radcliffe, Emma Watson, and Rupert Grint said their emotional goodbyes when filming on Harry Potter And The Deathly Hallows: Part 2 wrapped earlier this year.
But after looking at footage from a final scene where the three are meant to look like middle-aged parents, producers have realised that they actually resemble old people.
A source said that the re-shoot could cost millions.
'The cast and crew had a private giggle that the film has become Harry Potter And The Costly Hallows,' the insider said.
'They had all said an emotional goodbye to the filming they loved, so it was good news for them as they heard they had to come back.
'Their bosses haven't been laughing, though. They had to recreate King's Cross at Leavesden, which has been a huge expense. The professional term for the re-shoots is 'pick-ups' but everyone knows it's a bit more serious than that. It will be extremely costly.'