There are several types of errors recognized for coins, metals, and slot tokens. Here is a scarce $5 MGM Sands slot token from 1988 that I got from JamesC on eBay. The scan shows the error and the normal token. This is called a broadstruck token, as the metal planchet is struck outside the collar that adds the reeding to the edge of the token. This one is also about 10% off center.

The broadstruck errors are characterized by the lack of edge finishing and by the larger size of the finished token. This one squeezed out to about 2 mm oversize.

Other kinds of errors include: wrong metal, clipped planchet, rotated die, cracked die (this may be classed as a variety if many are made), struck through (something on the surface when struck), and filled die (something fills a small part of a recess in a die). The filled die errors can also be seen on some Coin Inlay Center chips, where part of the casino name or town lettering ends up missing.

Reputable mints will screen out errors and destroy them. If you want a cheap error token, though, look for one of the Horseshoe, LV tokens that have been in the hobby for a few years. Lots of these error tokens were not destroyed, and were sold to collectors for a couple of bucks. My guess is that someone took one of the trash cans and sorted out the off centers to sell. I've seen bins full of them for sale in Las Vegas.