Seemingly cruising to a second straight green jacket at Augusta National, Spieth knocked two balls into Rae's Creek in front of the par-3 12th hole and saw a 1-shot lead turn into a 3-shot deficit.
"Big picture? This one will hurt," Spieth said.
Spieth, the defending champion who led by five shots when he made the turn, made a quadruple-bogey 7 at the 12th. His tee shot came up short in the water, and then he failed to get his wedge shot from a drop zone onto the green. After another drop, he hit his 5th shot into the back bunker, from where he got up and down for a 7.
"It was a lack of discipline to hit it over the bunker coming off two bogeys, instead of recognizing I was still leading the Masters," Spieth explained.
"It was a really tough 30 minutes for me that hopefully I never experience again," Spieth said.
Nicklaus was one of many who took to Twitter to talk about Spieth's Sunday afternoon unraveling.
"I think the whole golfing world feels for Jordan Spieth," Nicklaus said in a statement shared on social media.
"My heart goes out to him for what happened, but I know that Jordan is a young man who will certainly learn from this experience and there will be some good that comes out of this for him. He's a wonderful talent and a wonderful young man," Nicklaus said.
Willett, who wasn't certain to tee it up in the Masters until his wife, Nicole, gave birth early in the week prior to the tournament, took the lead with birdies at the 13th and 14th holes before earning his first major victory.
The 12th has been home to plenty of misery over the years. Tom Weiskopf made the highest score in Masters history at the hole when he shot a 13 in 1980. Arnold Palmer found the same creek in 1959, and it cost him a shot at a second consecutive victory.
Perhaps the most notorious Masters meltdown was that of Greg Norman, which occurred 20 years ago. As part of squandering a 6-shot advantage at the 1996 tournament, Norman found the water on No. 12.
Willett became the first Englishman to win the Masters since Nick Faldo capitalized on Norman's collapse in 1996.