I’ve made a similar thread mid-2017, and it’s funny how many cards I was unsure about existing, I currently own.
To quote myself, and color the different cards:
green, if they do exist and I own them by now
yellow, if they do exist, but I don’t own them yet
red, if I’m at this point rather sure they don’t exist
orange, if I’m still not sure about whether they exist or not, even now…
So after 7 years, only the 2D 1st edition stamp on a Base Set yellow cheeks Pikachu; back stamp on a yellow cheeks; and grey stamp on a red cheeks I’m still unsure about whether it exists or not from this list above. Everything else I’ve either found or I’m confident by now it doesn’t exist - or have yet to find for my own collection, for those pesky German/Spanish Surfing/Flying Pikachu WotC promos…
As for a new list of cards I’m unsure about whether they exist or not, at this time of writing:
English Ivy Pikachu with upside down “GIOCATORI” stamp: I’ve heard rumors six were given to staff members of LPPCollecting, which I do kinda believe. But I have yet to see any pictures of them.
English Gym Heroes grey 1st edition stamps: I know grey 1st edition stamps exist for Base Set; Gym Challenge; and Neo Genesis, but I have yet to see any for the Gym Heroes set.
Japanese Base Set cards with small Commons symbol: I know the Holofoil Rare cards all exist with both large and small rarity symbol; and I know some of the Uncommons exists with both large and small rarity symbol as well. But I have yet to see any Commons with smaller rarity symbols. (Source: thread 1 and thread 2.)
Japanese 1995 Delta Playtest Sample Pikachu (jungle Pikachu with Ivy background): with all the different Sample cards we’ve seen lately, I kinda expected we’d already seen this one, which previously has only been seen in some CoroCoro magazines, but alas. But maybe it’ll emerge as physical card one day (soon). (Right one in the first picture here.)