Don't imagine it's just our hobby or just T206s.
The stamps I specialize in have lots of varieties.
One I just got has been known since at least the early 1960's, and was proven to be a constant variety in 2006. It's not listed in the Scott specialized catalog for US stamps. (actually a group of 4 different officials with a specimen over print where the "c" is broken. One per sheet and a short print run, so only 100 of each sold. )
On the other hand. Scotts lists a double transfer on the 10cent Treasury dept. It's been listed in the catalog since 1935, but there's
never been an article about it. And nobody has seen one, and unfortunately there's no record of where they got the information from to list it in the first place.
(Oddly I have a couple that I think might be it, but need to find a second copy to prove it. should be a long hunt.....
)
They also list or don't list plate varieties that should be on different printings, but the list them for one but not the others.
Big fun that cataloging stuff!
Steve B