Here's a YouTube video that shows how James Dowdell reprogrammed the ending of The Legend of Zelda to propose to his girlfriend Natalie Martin.
According to this Imgur post, James and Natalie like to spend time playing video games together. And so, when the two chose to play The Legend of Zelda on the original NES hardware, he used the opportunity to propose.
After setting a date to play the game, he fiddled around with the graphics to make Princess Zelda appear like Natalie.
James then hacked into the game's code, which according to James, was a lot harder than anticipated.
He then loaded the hacked game into an Everdrive cartridge to play it on the NES.
After playing the game to the very end (which took six hours), Natalie came across this screen:
Her response: “Of course I will marry you, stupid!”