Deliberate "mistake": At one point they use a Red Ryder BB gun (Yes, just like in A Christmas Story) and the guy says "You pumped it more than twice, didn't you" to his female counterpart when they shoot the bird. Red Ryder BB guns cannot be cocked/pumped more than once, due to the simple nature of the toy gun that it is.
Great sites
Mistakes
In the scene where Matthew McConaughey makes dinner to confront his parents and Sarah Jessica Parker about scheming to get him to move out his wine glass is magical. Before revealing he knows what is going on McConaughey stands up and drinks all of the wine in his glass and sets it back on the table. Seconds later, the wine glass has been magically refilled. And during his speech his wine glass jumps magically into his left hand before mysteriously returning to the table. See more...
Failure To Launch mistakes
Continuity: When Tripp has gotten ready for the day he comes down to the kitchen. We find his mother writing Tortilla Chips on the dry erase board. When Tripp finishes off the OJ in the kitchen he tosses it into the trash can opened by his mother. We see the dry erase board in a couple of different shots and the way it is written changes from shot to shot.
Continuity: When Kit and Paula are drinking champagne, the way Kit hold the fluted glass continually changes between shots. First it is in the middle of the glass, then near the top of the glass then back in the middle. As they sit down her grip is way down at the stem of the glass.