Jorge Lorenzo put his Austin MotoGP jump-start down to being distracted by mosquitoes on his visor as he came to the grid.
The two-time MotoGP world champion described the error as one of the biggest of his career, after his Yamaha shot forward long before the red lights went out to begin the race.
Having realised his error, Lorenzo chose to accelerate away as he had already incurred a ride-through penalty, which he took at the end of the opening lap before recovering to 10th place.
"When I arrived to the grid for the start I had too many mosquitoes on my helmet, so I decided to take the tear-off off and this confused me a lot," Lorenzo told the official MotoGP website.
"I was not thinking or being focused.
"When I saw the red light [come on], I thought it was the race start so I released the clutch and I made a jump-start.
"It was a big, big mistake. One of the hugest mistakes I've made in my career."
Lorenzo crashed out of the season-opener in Qatar on the opening lap so only has six points to his name after two rounds. That leaves him 16th in the standings, 44 points down on leader Marc Marquez, who is undefeated so far in 2014.
While Lorenzo underlined that Yamaha still needed more pace to take the fight to Honda, he admitted that his own errors were the most costly issue at present.
"For the moment we are not enough in general so we need to improve the bike," said Lorenzo, "but the biggest problem now is the two mistakes I've made, so I hope to not repeat this."
Yamaha team boss Massimo Meregalli said Lorenzo's errors still remained rare.
"No matter how hard you prepare for a race weekend there are some things that cannot be anticipated," he said.
"We all know he is one of the hardest working riders here and very rarely makes a mistake.
"He did a good job to make the best of it afterwards."