The performance of keyboard operators can be expressed in terms of keying time and errors; this paper deals with errors. If the causes of errors wore known, it might be possible to reduce the percentage of wrong keystrokes. Therefore, an attempt was made to identify these causes by classifying 293 errors, collected in a field study, into seven categories. About 25% of the errors wore duo to the operator misinterpreting input data; better data presentation may decrease this percentage. At least 40% of the keying errors could bo traced to underlying errors in finger movement control, and would not seem amenable to direct error decreasing measures. Automatic punching of repetitive information brings about numerous repetitive errors as well; improved instructions on the use of programmed punching facilities may reduce these errors.