A surprisingly easy mistake to make - especially if you are used to playing from a particular place or set of tees. On a strange course you can even play off the tee for a different hole.
This is a mistake that you must correct in a medal competition or you will be disqualified. The strokes that you have played from the wrong tee, or continuing with play of that ball, do not count, but you have a 2 stroke penalty and must start the hole again playing from the correct tee - playing 3. (NB In stableford you are only DQ'd from the hole if you don't play again from the correct tee).
In matchplay your opponent may choose to let your stroke stand, or ask you to play again (without penalty) from the correct teeing area.
Don't believe its an easy mistake? Jordan Spieth and Henrik Stenson got caught out this winter when the 17th and 9th teeing areas were moved for the last round at the Hero World Challenge.
