When Shay Stanley and Preston Waters are dancing in the water fountain, Shay leaves the table to go to the fountain wearing high heels. Later in the fountain and running to the car she is wearing sneakers. After she gets out of the car she is again wearing high heels.
When Preston goes to cash his million dollar check the owner of the bank gives him the money, and if you look at the bills they are all ten and five dollar bills. This means there were between 100,000 and 150,000 bills Preston would have to fit in his backpack. This is extremely unbelievable for Preston to carry; there's too many bills, and it would probably weigh a good thirty pounds as well.
In the scene where Quigley interrogates Beederman at the bank, you can see that Juice has a watch on his left arm. But at the end of that scene where Juice grabs Beederman his watch is on the right arm.
Preston's expense report towards the end of the movie indicates that he spent $999,667.83. However, after calculating the math and tracking the listed expenses with their appearances through the movie, the actual amount Preston had spent at this time totaled $504,879.08. Also the list does not include the amount that he paid for the house or the cash he had in the backpack. The maximum that the bottom should read is: $804,879.08.
When Preston calculates the time required to accumulate one million dollars from the birthday check, the answer is in error by several times. The movie shows the time as 342,506 years. If the interest is calculated yearly, it should take 338 years, or monthly it should take 331 years and 7 months.
Near the end of the movie Quigley learns that Preston has spent the entire $1 million dollars. But yet the check Preston cashes in reads August 9, 1993. This is a Monday. Later, Shay visits and asks about if Macintosh is available. Preston asks what day it is, and she replies Wednesday. Preston says no, but says he is. They go out that night and later Preston says that Friday there's gonna be a big party and Macintosh will be there. After the big party, that takes place on Friday the trio catch Preston and Quigley goes, "How could you spend $1 million in just 6 days." There is no way that 6 days could have elapsed during this time span. Only 4 days have elapsed.
When the guests leave Mr. Macintosh's party, they grab all the presents. A man grabs a present with blue wrapping paper. You can clearly see that it's an empty cardboard box and not even fully wrapped.