Right before Ian and Max drop the Icee on Wyatt and Gary, the cup is filled well over the top. Seconds later when they pour it, the level of the liquid in the cup has dropped considerably. The amount of Icee that hits Gary and Wyatt is much more than even the full cup could hold, yet after pouring it the cup is thrown down on their heads and appears to still have quite a bit in it.
When Wyatt and Gary come back from their first evening out with Lisa, they encounter Chet atop the stairs. When Chet first appears, his cigar is quite short. Later when Chet walks away and kicks his door open the cigar appears much longer.
Although the movie takes place in the fictional town of Shermer, Illinois, the police car that chases Gary and Deb is clearly an LAPD interceptor. However, the rear facing yellow lights should flash on a LAPD car. The ones shown burn solid.
The fake drivers license Lisa gives Wyatt to get into the bar would be rejected by any bar bouncer in Illinois. Besides being cut crooked, the drivers license number doesn't match the name, sex, and birth date information shown on the license. Bouncers know that Illinois drivers license numbers can be decoded to yield that data. The number shown on the license should belong to a female born in 1951 whose last name begins with P.
When Chet finds Gary and Deb in Wyatt's room, he hits them in the head with the barrel of the shotgun, and as it's not actually made of metal it clearly bends. Chet's hands also change position between shots.
When the cherry ICEE is poured over Wyatt's and Gary's heads, there is way more ICEE than could be held in that cup. Indeed, after the cup is thrown down afterward it appears to still have quite a bit in it.