Unexpected Sans refers to the sudden appearance of the character Sans from the popular indie game Undertale, often accompanied by his signature glowing blue eye and the opening notes of 'Megalovania.' This meme trend stems from the character's role as a surprisingly difficult secret boss in the game’s Genocide Route. Users typically place Sans into unrelated contexts or innocent scenarios to signal an immediate shift from lightheartedness to impending doom. Since the game's release in 2015, the meme has evolved into various formats, including jumpscares and 'bad time' jokes, cementing Sans as a permanent icon of internet subculture and gaming humor.