When Rachel is talking to Phoebe about the way she runs, her hands are on her hips when she is seen from the back, but when she is seen from the front her arms are crossed in front of her.
When Phoebe intercepts Rachel in the park when she's secretly out for a run, the guy in the gray sweatshirt ends up between them. However, in the next shot he is shown just catching up to them. Also, the one of the guys sitting on the bench in the background changes from reading a magazine to a newspaper.
When Chandler and Ross are trying to figure out where the couch goes, Chandler runs to the front of the coffee table. When he does that a camera is visible.
When Phoebe catches Rachel running in the park without her, the mic pack strapped to Phoebe's back is visible as she runs away.
When yelling through the door about Ross' guy problem, Monica asks if it's the same as Chandler's, and then yells that some of his medicine is still under the sink in the bathroom. But in The One with Frank Jr. (1996) it is shown that Monica has a pedestal sink with no vanity under it.