I'm partial to the one where a piece of hair or lint caused the inking issue in the 'o' of Washington!
It's surprising we don't see things like that more often. We've all seen lint in the air when the light is just right. That exists about everywhere including the print shops where stamps were made. And humans shed hair all the time. It's only where they are repeated over a period of time in the same place on a plate that they become a 'variety'. The one that comes to my mind first is the Scott 24 'Curl on shoulder' varieties. The one with a copper thread is interesting but a one-off type error...and who knows how it happened.
I've found similar 'errors' on Scott 295 where I have been able to locate several of the same error and others where I'm still searching for a 2nd copy of an 'error'. I would not claim to have discovered a 'new' variety though unless I can pinpoint it's position on a particular plate be able to demonstrate it having been repeated more than once.
These are not 634's but are inserted here to make my point.
One I've found more than once:
One I'm still searching for it's double:
Some refer to this as 'flyspecking' - I call it just another interesting part of the hobby!