Perhaps the most famous Mortal Kombat Easter egg is "Toasty!". It first appeared in Mortal Kombat 2, and would see programmer Dan "Toasty!" Forden's head appear on the bottom-right corner of the screen accompanied by a falsetto voice bellowing "Toasty!"
In Mortal Kombat 2, you'd get "Toasty!" sometimes after nailing an uppercut. But you'd also get it after performing Scorpion's fatality - the one where he rips off his mask to reveal his skull before breathing fire to burn his opponent to a crisp. Here's how it looked:
"Toasty!" following an uppercut was found in Mortal Kombat 11 back in April, soon after the game came out. But only last week was the Scorpion "Toasty!" discovered.
