How frequent are errors in blood typing?

Depending on the year and where the examination took place, the percentage of errors can be quite frequent.
Here is a study from 1995 showing an enormous amount of errors on the identity cards of Air Force personnel:

Our study is intended to determine if the blood types recorded on identification cards carried by U.S. Air Force personnel contain significant errors. We studied 262 active duty Air Force personnel. Eleven errors in blood type information were identified, including seven Rh type errors and six ABO typing errors. Such errors may arise when self-reporting of blood type is relied upon to create identification cards. The significance of these findings for wartime transfusion practices is discussed in light of current United States and NATO policy. Suggestions to reduce the error rate are offered.

Source:

ABO and Rh blood type errors on Air Force identification cards

See also:

4 major reasons why blood type tests can show different results