This highlights one of those significant challenges with flight testing: 'know your test article'. The challenge comes when aircraft that are used for STC type work are a 'Frankenstein' of mod state/STCs etc and the ability to fully understand the baseline standard and how that might affect testing, test outcome, mitigation etc is complex and challenging.
The other valuable lesson for all flight test professionals is the ability to be critical of unexpected results; to be able to pause, analyse and conclude why something didn't go as planned (failed). This also then needs any mitigation, risk reduction measures etc to be assessed for continued validity. Some similarities here with the G650 crash in 2011; unexpected results from previous testing (wing rock) that were not fully 'understood' and may have allowed a pause on testing to be instigated whilst a full analysis was carried out.