Accountability, or confessing when you make a mistake, is the mark of a professional. I didn't learn that lesson right away, but I'm glad I finally did. Your manager or customers might not be happy that you made the mistake, but they'll always respect and appreciate you for being accountable.
Getting caught trying to cover up a big mistake is usually a resume changing event. As trite as it may sound, the cover up is usually worse than the crime.
