Pianists' pitch errors were identified in a MIDI data base comprising more than 90,000 notes. Ten graduate student pianists had played four pieces (Schumann's Träumerei, Debussy's La fille aux cheveux de lin, Chopin's Prelude in D-flat Major, and Grieg's Erotik) three times from the score, after only a brief rehearsal. Pitch errors were classified exhaustively as substitutions, omissions, or intrusions. (A frequent form of intrusion was the "untying" of tied notes.) Nearly all errors occurred in nonmelody voices, often inside chords. The majority of the intrusions and nearly all substitutions seemed contextually appropriate. The repeated performances made it possible to distinguish consistent from unique errors. Consistent errors were more often omissions than intrusions, and consistent intrusions were more contextually appropriate than unique intrusions. Most errors seemed likely to be perceptually inconspicuous. This was confirmed in an error-detection experiment, in which eight pianists, some of whom had recently studied the test piece (the Chopin prelude), collectively detected only 38% of all objectively registered errors. Pitch errors, rather than being a categorical phenomenon (as a scorebased analysis might suggest), vary in the degree to which they violate the music, and their perceptibility is context-, listener-, and situation dependent. Members of a typical concert audience are likely to notice only a small fraction of a pianist's inaccuracies, which is in part due to the contextual appropriateness of most errors.