Spurred by the observation that force-distance profiles measured with the interferometric surface forces apparatus are often presented without error bars, we have considered the most important sources of statistical and systematic errors. We have also performed a calculation that yields quantitative error bars and insights into the most important parameters. In particular, we found instrumental drift as a subtle source of systematic error of unexpected magnitude-implying the exigency for clear error and drift declaration together with experimental data. Our results indicate that the spring constant of the force-measuring spring plays a pivotal role. Namely, for spring constants above some 10(3) N/m, systematic errors may result in misinterpretation of results. In typical instrumental designs, the spring constant is adjustable in a range from 10(1) N/m to some 10(5) N/m.