While Phoebe is giving birth to the first triplet, doctor says "Here comes the head" when you can actually see the whole baby and another one's leg behind the doctor's hands.
After Joey's kidney stones are removed and the doctor hands him the bottle containing them, you can clearly see there are Tic-Tac breath mints in the bottle. Kidney stones are typically yellow in color and misshapen like a spiky rock, which makes passing them so painful.
When Joey first starts videotaping Phoebe on the couch, watch the camera screen: Joey zooms out, and you can briefly see crew equipment before he zooms back in.