Undertale is a game jam-packed with secrets, to the point that players are still discussing them and hunting for new ones. They're even in official merchandise. Thanks to Dominic Tarason, I heard that you can cut open the official Toriel plush, dig through the stuffing and find a white heart. I did a little searching and found a video of someone actually doing it:
Weirdly unsettling! It's an apt Easter egg, though, considering what happens if you (SPOILERS) kill Toriel in Undertale. After a heartbroken speech borne of loneliness and concern for your future, she fades into nothingness, leaving behind only a pure white heart. Her heart, faced with the beating red of yours, cracks in half. You can watch that here, if you're a masochist:
Hahaha real funny Toby Fox and Fangamer. What a great gag lol. I'm definitely crying from laughter right now, and not for the complete opposite reason.