Revealing mistake: In the scene where J.D. first starts the bike portion of the triathlon, you can see his feet pedaling. You can see that there is no chain on the gears, allowing him to exaggerate the speed he is pedaling.
My Way Home - S5-E7
Revealing mistake: 5-07 - Doug clobbers JD with the fire extinguisher when he's in the body bag. When JD asks him why and Doug explains with the fire extinguisher in his hands, you can see him flexing it and that it's made out of rubber.
My Big Bird - S5-E8
Revealing mistake: When JD is kicked through the window by the ostriches, watch the window. It fractures about a half a second before JD hits it.
Revealing mistake: When Kelso hits Dr Hendrick's bike with his car at the end, you can tell it is not Kelso. While you can't make out his face, you can see that the man driving has dark black hair, while Kelso has light brown/grey hair.
Answer: No, it was a fake, they used a stamp.
It is real.