If you're talking about varieties such as doubled dies they are accidental. They occur during the die hubbing process. Things like the 1937-D 3 legged nickel that was an error that occurred while polishing of the die to remove clash marks. Varieties such as the Variety one and two Buffalo nickels and Standing Liberty quarters are intentional. They are often mistakenly called "type."
No, I'm talking about intended but minor design changes, like the date profile changes on the SLQ or the number of stripes in the shield in bust coinage, etc.
well, i do not consider them to be true errors, more like mint-employee "assisted" errors, or the same as a d carr coin, they are made on purpose, not an "error" of the normal manufacturing process...and each cud, or die break i slightly different, and i classiffy them as errors, i have a progression set of ike die failure coins i purchased at auction, each one is a "one-off"
Well a cent could get fed in between a pair of Ike dollar dies in a dollar collar theoretically on its own. An Ike struck with cent die on the other hand WOULD have to be employee assisted because the larger planchet would not pass through the feeding equipment and the feed fingers. It would have to have "assistance".
@Bmagold : Correct me if I'm wrong With this crowd, I don't know if I would have made that statement.