The One Direction hunk suffered the painful injury during an angling trip with his bandmates, and the incident was caught on camera for their new 3D documentary film, 'This Is Us'.
Director Morgan Spurlock told the Daily Star newspaper: "We were all fishing and Harry somehow managed to get the fishing line caught. First it was his finger, and then later in his backside. It will definitely by featured in the film."
Apart from the painful injury to Harry's buttocks, Morgan said the most difficult part of filming was the sheer number of obsessive fans who follow the boys everywhere they go.
The filmmaker laughed: "I've seen the lads literally get chased down the street."
The 'Best Song Ever' hitmakers are no strangers to being mobbed by fans, with Harry having to resort to drastic measures to evade his female admirers in New York last month.
The 19-year-old star was forced to get down on his hands and knees and crawl under a railing to stop being groped by fans when a gaggle of girls screaming to get a photo with him prevented him from moving.
Niall Horan has also said in the past: "I'm quite claustrophobic and I don't like everyone crowding around and shouting the same questions. I get a little bit scared sometimes. It's all good as we've got a good security team."