dress
Explanation
Two characters are at a party. One exclaims: "Oh my God, this is embarrassing! She's wearing the same dress as me!" The other character looks over and says: "She's... not?" The first character insists: "Can you COUNT? It's clearly a 4-holed sphere just like mine!" The caption reads: "This is why topologists never go to parties."
The joke is about topology, the branch of mathematics that studies properties preserved under continuous deformation (stretching, bending) but not tearing or gluing. In topology, a standard dress or T-shirt can be described as a surface with a certain number of holes (for the head, arms, and body opening). Two garments that look completely different in everyday fashion terms might be "the same" topologically if they have the same number of holes -- both are, in topological terms, equivalent 4-holed spheres.
The humor lies in the absurdity of applying topological equivalence to a social situation where visual appearance is what matters. The "wearing the same dress" faux pas is a well-known social anxiety, but the topologist character has redefined "same" according to mathematical criteria that no normal person would apply. The punchline -- "this is why topologists never go to parties" -- works as both a literal explanation (they would create these kinds of awkward situations) and as a gentle dig at mathematicians being socially oblivious. It is a companion to many SMBC jokes about mathematicians applying formal reasoning to everyday life with disastrous social results.