During the Boat Race, the radio-controlled submarine that is put into the water has no fair-water planes (the big ones on the coning tower) yet when they show it under water during one shot it has them.
In the beginning of the film Nelson is in a towel talking on the phone. In the matter of one line Nelson becomes fully dressed. There is not enough time for him to have buttoned the shirt and put on his belt.
When Sara and Nelson take the dogs to the beach, Sara has three dogs on a leash. Nelson has two. That makes five. They set the dogs free to run and for the rest of the beach scene there are only three dogs.
Regarding the bag of gifts "Santa" brought in through the window: the dishwasher seems rather heavy and large to fit into the red bag and we don't see him bring it in the window, or through the door like when he went to pick up Ernie. We also never see him plug in/turn on/prepare the tiny bubble maker, yet it is pumping out bubbles when he uncovers it.