Plot hole: When Martin decides to antagonize David by eating spinach, he eats it from his plate. When they both eat the spinach frantically, they each eat out of a large bowl. Where did the second large bowl come from, and why would there even be a second one if they only need enough for three people?
Plot hole: Hair does not contain DNA so the boy's mother could not be recreated from just a snipping of hair. The hair follicle contains DNA but that would only be present if the hair had been ripped out. The robot bear that gives David the hair didn't really have a chance to get any hair as he left with David early on in the film and they never returned. Added to that even if he did have the opportunity, he wouldn't have thought about getting extra hair. They only find out about the DNA cloning at the very end of the film when the mother had passed away a long time ago.