What makes it a mistake or a rare find

[NHMickey]

For the most part we all remember the Mickey in Drag pin right? The one of Minnie with no eyelashes... Well I can remember this has a hot pin (not so much now) but I also have a pin from Brazil that is half the size of the WDW one and it too is missing the eyelashes... So was it a mistake or did we all have rare pins? Also the Ying and Yang pins... Upsidedown and different colors... Mistake or rare?

So what do you think makes a pin or set of pins rare and not just a mistake?