In the flash back scene at Romy and Michele's High School which occurs in the late '80s there are cans of Coca Cola on the lunch tables. The cans seen are the modern cans of the late '90s with the elongated neck and push through top. The characters graduated in 1987 and the pull top cans with the tear drop shaped openings were still being used until 1989.
At the real reunion, after being caught telling a lie Romy says, "Now we're just a stupid joke just like we always were." One can clearly see by her lip movement that she is saying something else (possibly the 'f' word).
After talking to Heather, Romy makes the next woman wait. When the woman's back is to the camera, her elbow's on the counter, her chin being supported by her hand. This is not so when she is facing the camera.
As Michelle is getting back into the car when they are in the desert fighting, there is a coke cup on the center console. When they drive away, there is no cup on the console and neither one of them is holding it.
At the "dream" reunion, the Mercedes limousine changes from one with two moon roofs, then with a shorter limo with one moon roof, and back again. When Romy is hit by Sandy's limousine, the rear window is blacked out by window tint, but the tint disappears moments later when Romy and Sandy are shown in the back of the limo. When the cars appear inside the reunion, Sandy's limousine now has blanked rear door opera windows and a smaller back window (additionally, the trunk mounted television antenna that Romy knocked off before entering the reunion has now re-appeared). As the limousine exits the building, it is back to the longer, dual moon-roof model seen earlier with the trunk-mounted television antenna missing again.
In the end of the movie where Romy and Michelle are lifting off in the helicopter, when they do an aerial shot of the class watching them lift off, you can see the shadow of the camera crew in a lift box.
Romy and Michele attend their tenth year high school reunion (they graduated in 1987) and try to impress their classmates with the lie that they had invented Post-it Notes, a product that was widely available by 1980. Even though these characters weren't very bright, surely one of the classmates would have caught on to such a ridiculous fabrication.