Was scrolling through FTR for the first time in months and saw this thread. This pic and variations of it have been making the rounds lately. It's an offshoot example of what's called Russell's paradox, which is kinda sorta somewhat important to know about in set theory. Long story short, it's a kind of set contradiction where you define something that cannot exist based on the definition. A related example would be deciding if, "This sentence is false," is true or false.

The mathy part of it is kind of long and drawn out, but a fun consequence of the paradox mentioned above is that if you want to avoid it, you can't define a set as any collection of objects (which is how most traditional math classes define a set).