Error cards typically have some kind of misprint on them, which is rare and, therefore, potentially valuable
Biogenic Ooze - Not only does this make a token when if flickers, but it also makes them all bigger over time, and can even make more tokens even if you don't have flicker options
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Rare cards, secret rares, and holofoil cards tend to be more valuable, and their respective symbols help collectors distinguish them from more common cards
There is notable a full-art foil Professors Research Supporter card featuring the Gen Five Professor, as well as card sleeves and a deck box
Im also including the popularity ranking of each Doctor on EDHREC as a comparison