When Randy and Molly are in the kitchen talking, Molly is boiling hot water in a Le Creuset tea kettle which whistles, even though the "whistler" isn't in the spout. Also that model of tea kettle has a harmonic hum not a whistle.
When Neil and Dupree are watching the game at Carl's house,
Carl gets home, opens a bottle of scotch and pours himself a glass. When he approaches Dupree he is holding the opened bottle, then the cap is on, then it changes to the glass of scotch that was on the counter, then back to the capped bottle.
After Carl and Molly kiss at their wedding they turn to walk back up the aisle with Carl on the left and Molly on the right. In the next shot they have changed sides with Molly on the left and Carl on the right.
When Dupree is playing baseball with the neighborhood boys, he goes and talk to one of the boys where you can see a UPS truck. When he goes back a second time, the UPS truck is gone. The third time he looks over at the boy, the UPS truck is back again.
When Dupree makes breakfast for Molly and Carl, he puts a large yellow bowl (the one he used the day before to catch the water from the overflowing toilet) on the breakfast table. In the next shot, the bowl is gone.
Near the end when Dupree jumps off the curb and hurts his leg or ankle, the closeup shot includes a large pole of some sort coming out of the lawn above the curb, whereas nothing was there in the wider shot.
When Molly and Carl are arguing in their bedroom about Dupree having changed the message on their answering machine, the purse on the dresser changes from a pink bag sitting upright to a yellow bag laying on it's side.
When Molly goes down stairs because she can't sleep she stumbles upon Dupree on the bean-bag, she drops both glasses. You can only hear one glass break and in the next scene she is still holding the glass that she dropped from her left hand.