There’s a scene in Harry Potter and the Sorcerer’s Stone where you can clearly see a stuntman double in the place of Rupert Grint, who plays Ron Weasley. But, the stuntman is only in the movie for four frames, so it’s literally impossible to see him unless you freeze-frame the DVD.
The scene where the stuntman appears is when Ron has just declared “check” in the chess scene, and is about to be thrown off his steed by the Queen.
Right before the Queen attacks, we see a close-up of Ron, clearly Rupert Grint.
In the next shot, the Queen attacks.
In the very next shot, as the Queen’s lance hits Ron’s steed, you can clearly see that it’s now not Rupert Grint, it’s a stuntman, dressed in the same clothes.
Here’s a zoomed-in image of the above frame:
Smoke from the steed exploding obscures the face of the stuntman three frames later. Since a movie plays at 24 frames per second, that means the stuntman is on the screen for about 1/6 of a second. That’s why you can’t see him without freeze-framing the DVD.
Just 12 frames later, just a half a second, the very next shot is a closer shot of Ron falling backwards off the steed, and now it’s clearly back to Rupert again.
It’s curious there was a stuntman for this shot, since Rupert clearly shot the scene falling off the steed as well. They obviously shot it multiple times with both the stuntman and Rupert.