A player reaches the 3rd hole where the ground of the teeing area is uneven and not level. The player tees the ball in the flattest part of the teeing area but when taking a stance one of the tee markers interferes, so the player reflexively reaches down and moves the tee marker. Before making the stroke, another player in the group says that moving a tee marker is not allowed.
What happens now?
The teeing area rules apply whenever a player is required or allowed to play a ball from the teeing area. These rules allow for certain conditions in the teeing area to be improved, such as altering the surface of the ground. However, tee markers are treated as being fixed and there are restrictions to when they can be moved (e.g., considered movable obstructions.)
Unfortunately, in this scenario, the player is not permitted to move a tee marker – but, within the new and more forgiving rules – the player may correct the mistake of moving the tee marker before making a stroke. If the mistake is corrected (and the tee marker is replaced to its original position), there is no penalty. (The player is also allowed to move the ball to another part of the teeing area, without penalty.) If the player made a stroke without replacing the tee marker, the general penalty would have applied.
For more information, key rules references are R 6.2a, 6.2b(3,4), 8.1a and 8.1c(1).