Hobby knowledge grew by leaps and bounds in the last 20 years, leaving remaining unknowns in a realm of the completely obscure and dismissible. Imagine a kitschy photo paper that started blank, but revealed pictures if you rubbed water (or spit) on it and let it dry in the sun. Would you keep them around? That's the rare (and still mysterious) Topps Hocus Focus.
Card panel front
Bob Lemke, editor of our hobby's annual Standard Catalog, recently wrote about two final Hocus Focus discoveries, #42 Hal Smith and #8 Mel Parnell, that turned up in a group lot with HOFer Jackie Robinson. Baseballers make up only a portion of this 120+ subject set and it took 55 years to verify that one 18-player checklist, so you can imagine (The last one on Ted's strip reminds me of my favorite palindrome: "A man, a plan, a canal: Panama.")
Card panel back
Hocus Focus collectors continue to publish their research into this ugly duckling set on forums like Net54Baseball.com, including this essential thread of scans and publishing schedules. It appears Topps developed (no pun intended) the "water and light" photo effect over several years, with 1956 as its apex--or at least their final set based on it.
Not sure if buyers would take to this photo technique in the 21st century, but Allen and Ginter's always full of surprises. Would you be interested in Topps bringing back their spit-developing cards as a gimmick?
Value: Complete, well-developed Hocus Focus strips cost several hundred dollars and up, with individual star players running somewhat less. (Due to scarcity, you're more likely to find baseball singles at auction than on eBay.)
Fakes / reprints: This set's valuable enough to fake, but relatively low-demand. Check out the Net54 thread for info on each set, so you know how to distinguish them.