This seemed like quite a rollercoaster ride that could have been avoided by better monitoring (monitor the config change, monitor the FMA etc). I am absolutely no skygod, heck, I'm definitely worse at manually flying a plane now then 6 years ago when I flew a plane without autopilot. But previous experiences on that old plane have instilled a fairly strong habit of monitoring and cross checking in my flying, and it helped prevent a similar incident maybe a month or two ago. I asked for next step of flap extension, FO raised flaps by one step instead. Caught it within a second or two luckily, reselected the desired flaps, mentally cursed at the FO for a split second, continued the approach as all was still in good shape and stable.
After shutdown at the gate I asked the FO what happened but no clear explanation or reason for the flap retraction was available. Normally a decent pilot, just seemed like a massive brain fart. Anyone recall the Luxair Dash-8 gear retraction on rotation? One of the conclusions there:
The early, not requested, grasp of the landing gear selector lever during the take-off callout procedure came suddenly and unexpectedly for the PF. After the event, the co-pilot could not explain her actions.
Publications on human performance and error management (see page 16 ff) describe such actions as Slip. It is a spurious action which occurs unintentional und unplanned in a correct, known, often trained and repeated course of action. Especially processes which are repeated quite often and therefore generate reduced concentration are susceptible for these kinds of errors.
And of course, an often recurring theme, some of the issues in this Wizz incident could have been prevented by reverting back to basics. The detents are there to help but you still have enough thrust lever travel assigned to manual thrust... Use it, AP off, AT off. Reselect once stable.