Blunders are usually caused by human errors. Blunders are mistakes in the processing of taking, reading, transcribing or recording survey data. Blunders are the most difficult errors to deal with because they are inconsistent. For example, if you read the wrong end of the compass needle, the reading will be off by 180 degrees. If you transpose the ones and tens digits on tape measure, the reading could be off by anything from 0 to 90 feet.
Blunders are extremely common in cave surveys.
By extrapolating I have concluded that in surveys like these, there is at least one blunder in every 40 shots. Paul Burger did careful study of blunders during of a recent resurvey of the commerical section of Cave of the Winds. He found that one in 20 shots had a blunder. These figures are impressive, because these surveys were very carefully done in relatively easy, well lit, concrete-trailed walking passages. In more difficult caves, the blunder rate goes up rapidly.