Given Phoebe's staunch vegetarian lifestyle and opposition to fur, it is unlikely she would want leather pants.
When Ross, Chandler, and Joey are in the guys' apartment talking about who will be best man, the camera is on Chandler as he says, "Oh, it's awkward" and you can see Joey stand up from the recliner behind him. But in the next instant, Joey's seated again.
When the vet brings the ring into the waiting room, the large black and white dog behind the guys alternates standing and laying down in between shots.
When Joey tells the stripper she can play with his duck, a close-up of the duck is shown with a few white feathers on the floor. In the next shot, the feathers are gone.
Phoebe says that it's easier having 3 babies playing "Bringing in 'da Noise, Bringing in 'da Funk" on your bladder. The correct title of the musical is "Bring in 'da Noise, Bring in 'da Funk".
After the bachelor party, when Joey unlocks his door and is demonstrating how the duck is trained, you can see where the set ends on the bottom left corner.
At the Coffee House when Joey and Ross are discussing who to invite to the bachelor party, after Ross says only to invite the 'dinosaur dudes' Ross looks directly into the camera.
During the end credits scene, each time the duck quacks, its beak remains closed each time.
Directly after Ross yells at the stripper, Matt LeBlanc (Joey) breaks character by ducking down behind David Schwimmer (Ross) and visibly attempting to stifle a laugh with his hands.
When they are reminiscing about the duck, there is a scene where Chandler is in the boat and the duck is in the bucket, and you clearly see a member of the crew and the end of the wall.
At the coffee house at the beginning, you can tell it's not Jennifer Aniston sitting at the table. It appears to be her stand-in.