Jon Rahm -- the world's best golfer -- missed a 10-inch putt during the first round of the Arnold Palmer Invitational on Thursday ... and it was truly unbelievable.
Rahm went to nonchalantly tap in his par putt at the Par 3 7th hole at the famous Bay Hill Golf Course in Orlando ... when disaster totally struck.
Rahm didn't hit the ball hard enough ... and according to PGA Tour's shot tracker -- it actually went just 1 inch!