Audio/visual unsynchronized: When Kate allows Joe into the house to use the bathroom, we hear her say "I'll show you where it is," but her lips aren't moving; she's just smiling as she closes the door.
Continuity: When Sara originally makes up the "contract" between herself and Joe, she does not sign her name on the bottom, writing only "One Million Dollars". Later on when Joe shows her the contract, her name is signed on the bottom.
Continuity: There are many indications that the movie takes place around Christmas time, but near the end of the movie, as it is approaching 7pm (Texas time), it is still daylight outside, when it ought to be dark.
Continuity: Joe's weight changes throughout the movie, as can be seen by the size of his neck. Matthew Perry's weight change was due to his drug addiction that occurred while filming (see Trivia).
Continuity: After Sarah is served by Tony and Kate, we catch a glance of the Polaroid picture in Kate's hand and it has a large Texas Flag on it that is not in the room.
Factual errors: The legal goofs in this movie are hard to count, but to start with, in New York, the rule is that when serving process on someone all you have to do is mail it to them and tack it on their door. Sara running would have no legal effect.