A perfectly ordinary right triangle. Legs of length 1 and i (the imaginary unit). Hypotenuse, by Pythagoras: sqrt(1^2 + i^2) = sqrt(1 + (-1)) = 0. So this is a triangle with a hypotenuse of length zero. Which is also an extremely degenerate triangle. Which is also fine, because we never said it was real. For mathematicians, complex-analysis survivors, anyone who has lectured undergraduates that "yes, geometry breaks when you go complex, that's the whole point", and anyone who appreciates how much chaos a tiny i can cause. Minimal line art — three lines, a right-angle marker, three italic labels. No words. The diagram is the punchline.