Rory McIlroy was 4-under through 14 holes Thursday at The Masters, but once again, he made a massive mistake that took the wind out of his sails.The Northern Irishman made a double-bogey seven on the par-5 15th after he chipped it off the green into the water.A mistake like that is what so many have grown to see from him at Augusta National. However, most of his first round felt different, until it didn't.Patrick Cantlay recorded an eight on the same hole a few groups ahead of McIlroy, so the four-time major winner was not the only one to make the mistake on Thursday.Regardless, it was a tough blow for McIlroy. He hit a great shot off the tee and had 241 yards to the pin. McIlroy went long on his approach shot and tried to hit a small chip onto the green.Instead, his third shot rolled past the hole into the water.Eliminating big mistakes is a key to winning at Augusta National.He had such an impressive round, so it deflated his round when the 35-year-old made that double.A seven is the worst score on any hole by a Masters champion, and the last time that happened was Craig Stadler in 1982, per stat guru Justin Ray.McIlroy could derail and go another year without completing the grand slam or dig deep and recover. However, after his double on 15, he made another double on 17 to sink to even par on the day.It was a disastrous finish for McIlroy, and it could be another one of those moments that sit with him a while.More Golf: Masters Winner Fred Couples Makes History, Sinks Epic 191-Yard Hole Out