When Rick is sitting outside the coffee shop talking to Leigh, the same extra walks by at two separate times during their chat. Same entry and exit point on the screen both times. It was a woman in a purple top and blue jeans with a purse.
When Rick pulls up to the beach house, the driver's side of the van isn't vandalized. In the next scene, outside with Fred and Grace, it is vandalized again.
When Grace is in the bedroom with her flirt Gerry, his shirt is open when he comes from the bathroom, then buttoned when they have a talk. Then, he stands up and closes it.
When Rick goes into the gym to sign-up, the assistant gives him a clipboard for his details, but there is one already there (possibly from a previous take) which he goes to fill in instead. In the next scene, there is only one clipboard.
When they are in the park talking before going into the coffee shop, Owen Wilsons characters shirt tail is untucked while walking to the bench. When he gets up, it's tucked in. When he turns around to leave it's untucked again.
When Brent is vandalizing the minivan, he smashes the front windshield, and it shatters. However, windshields are made of laminated safety glass and are designed to not shatter.
After Brent fires his last shot, his gun's slide should have locked back empty.
After golfing when driving on the road in the golf cart they stop at an intersection and wait for a green light. When they start to drive you can see the crossing indicator behind them turn to walk indicating they start to drive when the light just turned red instead of green.
The splattered mess on the back wall of the shower-stall appears a good second before the lady audibly farts, and a very noticeable moment before she even leans forward fully to sneeze (and simultaneously fart).
Directly after the bathtub shart scene, you can see that someone is rearranging the woman's skirt from behind.
When Gary is arrested (in his dream sequence) he demands to see "a barrister". In England, barristers do not represent clients who are under arrest at a police station (a solicitor does that).