After reading all the posts, I feel my answer of "mental mistakes" leads to bad drives, poor ball striking, missed puts and quite simply all the dumb mistakes we make as weekend warriors that cost us several strokes per round. I've been playing for a while and have noticed over the years that most golfers just get up to the ball and hit it. No practice swings, no alignment checks, no visualizations, no pre-shot routine whatsoever. I see this all the way up to putting. I don't play slow, but I do take my time. Why not take a second to line up some writing on your golf ball with the putting line you want to roll the ball on? So many golfers don't take the time to apply the mental aspect of swinging, hitting and scoring in golf to understand the impact this will have on the outcome of the shot.
For me, sticking to my pre-shot routine is paramount to executing each shot. Playing with people I don't know makes me self-conscious about the "appearance" of taking too much time to hit a shot. Therefore, I rush the shot and or the routine. I can hit the ball like a pro on the driving range, but my mental lapse on the course is what's costing me anywhere from 3-6 strokes a round.
Who supposedly has the strongest mental game in golf? TW. Who's the number one golfer in the world? TW. Who's mental game has cost him at least two US Open's? Phil. Rory's mental game has cost him the number one spot and may have ruined his 2013 season. Just saying, all these pro's are excellent ball strikers. It's their mental mistakes that makes the difference.