Reciprocal transfer functions don't seem to be reciprocal
Louis Dartez discovered that two reciprocal transfer functions don't match well across all frequencies. There are a number of frequency bands that don't match.
Motivating example is in the xml file linked to in this alog:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=70705
compare these two channels
H1:GDS-CALIB_STRAIN_CLEAN
H1:CAL-PCALY_RX_PD_OUT_DQ