Continuity mistake: Right after Steve rescues Mary from the run away horse, for a split second, you can see the foreign guy's horse in the left hand corner of the screen. In the next shot, you can see that the rest of the riders, including the foreign guy, haven't caught up to Steve and Mary yet.
Other mistake: When Steve rescues Mary from the galloping horse, after he picks her up, in the far shot, it's quite noticeable that it isn't Mary wrapped around Steve, it's a dummy.
Continuity mistake: On the day of Steve and Fran's wedding, the part in Penny's hairdo switches sides from shot to shot.
Continuity mistake: In the runaway dumpster scene, the film switches back and forth between a yellow cab going uphill and a red cab going downhill.
Continuity mistake: In the first wedding in the film, the two women who are talking about what a romantic life the wedding planner must lead are sitting. However, the shots before and after their discussions show the entire church standing because the bride is walking down the aisle.
Continuity mistake: When Mary and Steve are sitting in the park watching the old movie, Steve starts tossing the colored M&Ms out of his hand and all that's left are the brown ones. A couple shots later you can see that there are colored M&Ms still in his hand, but he did not take more.