Is the "comma pump" an empirical problem, or merely theoretical?
It's a standard music theory exercise to show that certain chord progressions don't actually quite work. For example, in the I-IV-ii-V-I progression, following the "naive" method of finding each pitch (tuning each major chord as a pure 4:5:6 ratio and the minor chord as a pure 1/6:1/5:1/4 ratio, and keeping common notes between adjacent chords constant) will lead to the final I chord being flatter than the initial one by a syntonic comma.
Is this actually part of the reason why choirs or singers singing a capella (or anyone else making music without a pervasive absolute pitch reference) tend to drift in pitch unless they're careful and/or skilled? Or is anyone whose pitch sense is good enough for this not to be dominated by other errors also capable of instinctively compensating for it?