In one scene, Kevin burns a CD to play Marv's voice back at Vera, when in reality, he would have used an audio cassette, because this movie takes place in the 1991, and burnable CDs were not released to the public until the late 1990s.
Design in the early 90s was usually edgy, not round. The movie therefore contains items from different parts of the 90s era or not from the 90s at all. Some of the items that don't fit the time frame are a flat screen monitor, a cell phone, a game console controller and steering wheel, a Hi-Fi mini-system, an AV Receiver under the gaming center, a computer keyboard next to Kevin's bed, and a minivan. Also, Kevin's PC is way too fast (286 or 386 that time).
A third-generation Dodge Grand Caravan is used in the movie. Although it was designed around the time the movie is set in, the year 1991, production of this generation didn't start until late 1995. In 1991, the second-generation of the Caravan was new and had just entered production in late 1990.
When Kevin floods the lower floor, he is seen standing at the front door and then picking up items floating around. At no point do you see Kevin fall over or get wet. However When Dad calls Kevin for a "talk", his bottom is all wet.
When Natalie walks in during the flood and begins to scream, Kevin is standing on the first step of the stairs with his shirt uneven, and the strings for his pants are showing. In the next shot, Kevin's clothes are neatly ironed, his shirt is even, and the strings have disappeared.
After Vera gets insulted, hearing Marv's Voice she shouts "Door open", we see the door opening and Vera entering, then we see Marv just crawling out from underneath the fallen wardrobe. Then in the next shot we see the door opening (and Vera entering) again, with Marv walking up to it just before Vera slaps his face.
When Kevin escapes from the wine cellar using the dumbwaiter, he goes up. Then, he sends it down onto the robber's head. Then, the female sends it up, and then back down again when it hits the male robber's hand. But, when Prescott goes to the dumbwaiter chute, the dumbwaiter is up instead of down.
On Christmas morning, Kevin fiddles with the presents. When his father says he has to go and pick up the royal family, look at the presents over Kevin's shoulder. The view changes so it is by the Christmas tree, and the same presents are in a different position (especially the dark metallic green one and stripy one).
In the flood scene when Natalie walks in and begins to scream, Kevin is standing on the first step of the stairs with his shirt uneven, and the strings for his pants are showing. In the next shot, Kevin's clothes are neatly ironed, his shirt is even, and the strings have disappeared.