"Is this an error?" Many times when people ask this question, they are jumped on by people who assume that they are buying Hot Wheels for money. Sometimes this is true, but most people are simply interested in whether the item is unique or has trade value.
On the question of rarity, most production line errors are indeed rare, at least on an individual basis. I would be surprised if there were more than half a dozen Blimps packaged pn an Oscar Mayer Weinermobile card, or Auburn 852s with one wheel painted on the wrong side, or Speed Gleamer T-Buckets with missing engines, and any error may be completely unique. There are categories of errors that are common - mispackages, upside down cars, and mismatched wheels are all well-known - but considering the number of Hot Wheels produced each year, there are really very few errors. That's why they are special to error collectors.
Errors typically do not sell for a lot, no matter how rare they are. In many cases, sellers try to pass off errors as "rare variations". Sometimes, this is an honest mistake and sometimes it is an attempt to increase the price of the item. A rare variation can be worth a lot, while an error, no matter how unique, may command only a small premium. Don't be fooled - know your variations!
My personal value system for errors is that they are worth only a little more than the same car without the error - maybe $1 more. There are some very interesting errors that might be worth a bit more to me, such as a car that really looks weird due to the error. For a mispack or upside down car, I won't pay any extra, so I usually end up getting these from kind people who send them to me.
In most cases this extra value requires that the item be in a blister pack and that the blister pack appear to be unopened. There are some cases where I will forego this, as in the case of the Cusom AMX with the unpunched rivet, but it has to be something where the error would be hard to fake at all.
5-pack errors are pretty much considered to be worthless. The main reason is that it's considered too easy to fake these errors.
Of course, there's always eBay. I rarely get error items from eBay because there are people who will pay more than I will. It's not that I'm cheap - I just don't like the idea of contributing to the market for errors.