Wikipedia wrote that T206 is a tobacco card set issued from 1909 to 1911 in cigarette and loose tobacco packs through 16 different brands owned by the American Tobacco Company. Collectors refer to the set as “The Monster” since it’s a 524 card set, but with 36 different backs advertising different Tobacco brands, an enormous number of cards are needed for a “complete” collection. The most famous (not rarest) card from the set is the Honus Wagner because of its limited distribution numbers and Wagner’s popularity as a player and Hall of Famer.
Keith Olbermann positioned a few new theories about why the card is so rare. Some believe that the printing plate broke early in the printing process, others suspect a copyright issue, and others think Honus Wagner pulled the card because he objected to its printing (maybe not wanting to encourage kids to smoke). Olbermann proposed that perhaps the Wagner card was a mail-in giveaway, and that’s why only between 50 and 200 cards were ever potentially distributed. Estimates today are that less than 60 of the cards exist to this day of the original 200.
The T206 Wagner currently occupies 3 of the Top 10 highest sports card sales of all time. And Goldin Auctions recently sold a PSA 2 variant for $3.66M. If more of them came to market, I suspect they could occupy many more places on the Top 10 list.