Given that there are so many different types of wheels, and given that the wheel assemblies are often the smallest parts on a car, it's no wonder that the most common car errors are related to wheels.
First up, we have the simple wheel mismatch, in which one or more wheels don't match the others. There are two basic types of mismatches: single-wheel and front-rear. Since it's usually very hard to see the side of the car next to the card, it's hard to tell which kind of mismatch a car has, and in cases where there's a single-wheel mismatch next to the card, it may never be seen (I don't know of an example of this, but I'm sure it exists).
Front-back mismatches seem to be caused by changes in the production line. There's probably some "hopper" that doesn't get emptied out properly, and as a result, there are usually several collectors with the exact same error car.
Here are examples of wheel mismatches:
Some people have found cases where a car has the wrong size (but correct spoke configuration) wheels on the front or back of a car. Except in cases where the front wheels are bigger than the rear wheels, these are almost always variations.
On occasion, a wheel gets put on backwards. Again, due to the difficulty in seeing them, around half of these errors are unknown until the package is opened. I know of no cases where more than one wheel is put on backwards, but I'm sure they exist.
Here are examples of backwards wheels.
Here are examples of cars where one wheel is missing the chrome:
Here are cases where wheels are missing from cars:
Here's an odd case where there is a wheel in the package, but the axle and other wheel are missing:
