Last spring a QC analyst called me about a failing USP <621> assay on a compounded cream. The method was validated, the column was new, the standards were fresh, and the system suitability on the previous shift passed. The next morning, three injections in a row came back at 92 to 94 percent of label, with an RSD that drifted from 0.8% to 2.6% across the run. Nobody had touched the method. What had changed was that the vial tray had been reloaded by a different technician, and the aut...