We asked school- and college-aged instrumentalists (N = 32) to imagine an ideal performance of a brief passage of music, record a performance of the passage, and describe discrepancies they noticed between their imagined and actual performances. The more experienced participants took at least as much time to imagine their idealized performances as it took to perform them; less experienced participants took less time to imagine what they were about to play. There were no differences among experience levels in the numbers or types of discrepancies identified. The differences between more and less experienced participants were also evident in a subsequent practice period. More experienced musicians' practice included more frequent moments of pause, whereas school-aged participants seldom paused and tended to address performance issues other than those identified in their commentaries.