What happens if the wrong score is on my card for a hole?

It depends which way the mistake goes (Rule 3.3b(3)), once you've returned your card:

Exception for penalties you didn't know about: if your score was too low only because you left out penalty strokes you didn't know you'd incurred, you're not disqualified. If it's found before the competition closes, the Committee adds the missing penalty strokes to that hole. This exception doesn't apply if:

• the missing penalty was disqualification, or
• you were told a penalty might apply, or were unsure and didn't raise it with the Committee before returning your card.

Mistakes in adding up or in handicap strokes aren't penalised. You don't have to add up your score or apply your handicap; the Committee does that.

The best protection: check each hole with your marker as you go, and go through the card carefully before you sign and return it.