When someone makes an error on an item before it goes into production, this is called a "pre-production error". Because of when they happen, pre-production errors are often made to many cars, so these errors are very common.
The most well-known pre-production error is the 1996 Treasure Hunt Dodge Viper RT/10 in red with silver lace wheels, which is shown at the top of this page. Approximately 60,000 of these were made and distributed, and this car is still controversial. Stories on what happened vary - even Mattel gave different stories. It is important to point out that this was not a case of Mattel making 60,000 extra cards in 1996 - the cards say "Made in China", and Mattel wasn't making cars in China when the original Viper was made.
Another widely-discussed pre-production error is the #473 Street Beast, whose
tag line is "50s Retro Original".
The "error" in this case is not the name. Despite the fact that there are other cars named Street Beast (including a Tattoo Machines Corvette from 1993), this appears to be intentional. The name change corresponds with the release of the Street Beast series in 1997. The real "error" is in the tag line - it appears that someone just forgot to change it when they grabbed the printing information from the database.
A similar situation is the #601 Commando, which is tagged as having "Tandem Machine
Guns", which appear to be missing, but which are in fact not supposed to be
there in this version.
Another well-known pre-production error is the card which contains the wrong
name for the car due to a tampo change.
For example, the #72 School Bus card can be found with a police bus.
This card was changed to say simply "Bus".
A better example is the #577 Police Cruiser card with a Fire Chief car.
The casting for the Fire Chief car is the same as the Police Cruiser - they
just forgot to change the name to match the paint job.
One of the few pre-production errors that isn't a package error is the "FISRT AID" version of the truck from the Baywatch 5-pack. A picture of this is forthcoming.
One of the most interesting pre-production errors is actually a triple-error.
The 1998 First Edition #7 32 Ford shown below has a package that says "#7 of 48"
(there were only 40 FEs in 1998),
the plastic insert is for the Slideout, and there is a metal post hanging from
the ceiling of the car.
Now, it should be noted that the "48" was actually the number that was planned
at the time the cards were printed, so this might not technically be called an
error.
Also, the Slideout insert may have been used on purpose, since it does fit the
car, and it appears in later versions of the 32 Ford, as shown below:
By the way, the sometimes-ridiculous wheel choices are not considered pre-production errors - just lapses in judgement...