Blundered refers to the fact that a mint employee made a mistake in punching a letter or digit on a coin. It can be a date digit, denomination digit or a letter, and sometimes part of the design. Usually these are corrected but you can still see the blundered digit or letter on the coin.
Repunched dates, misplaced digits and letters, inverted letters, digits and mint marks (RPM's) are examples of error or varieties that are blundered letters and digits. There can be some confusion with tilted mint marks, so a tilted mint mark is a mint mark tilted more than 45 degress but less than 90 degrees and a blundered mint mark is tilted or rotated 90 degrees or more.