When the Principal looks through his Oldsmobile left outside rear view mirror at the boys in the shopping cart, he is looking through a larger than stock mirror that is mounted to the door with a bolt. In other scenes the Old's mirror is a stock body color sport mirror.
When Harry and Lloyd cover the car at the convenience store in gasoline, a Bell Atlantic payphone is in the background, with a blue and green logo. In 1986, the payphone would've had a New England Telephone logo. The company became Nynex in the early 1990s, and merged with Bell Atlantic in the late 1990s.
Mrs. Heller suggests that they base the Thanksgiving Parade float on George Washington crossing the Delaware. George Washington crossed the Delaware river around December 24th, about a month after Thanksgiving.
When Jessica is chasing the little school bus to the museum in her station wagon, the car's right hubcap is missing. Later in the chase the hubcap reappears, and then flies off when hitting the ground after her car becomes airborne.
At the start of the parade float designing scene, Ms. Heller underlines the word FLOAT? on the chalkboard. In shots of the chalkboard afterwards the line disappears, reappears and sometimes the line is obviously shorter.
When Lloyd first notices the short bus, on the right front tire, the reflection of the sun is at 12 noon, and there is no shadow from the side view mirror. When they approach the bus, the sun changes to about 2 o'clock on the tire, and the mirror casts a shadow on the bus.
When Harry and Lloyd fall from the ceiling, they walk down the school hallway and under an arch as they discuss the upcoming date. When the camera cuts to behind them, they are back where they fell out of the ceiling, with the arch still in front of them.
When Lloyd finds Harry's "treasure chest", it's is lying flat against the wall. When the principal and the lunch lady discover it missing, you see is a small dirt outline, as if the chest was pointing out from the wall, instead of larger outline, as if it were flat against the wall.