I'd say this wasn't the first time that the props were feathered but the mistake was spotted in time, I'm quite surprised that there isn't a audible warning that they have been feathered, looking at the position of those levers, especially with flaps 15 selected and when flaps 30 are called for then IMO its almost inevitible that someone will, under the "right" conditions, pull the wrong lever(s).
The Staines accident occured despite having three pilots, a P1 and a P2 plus additionally, a monitoring pilot, a P3 sitting back between them. After this accident a speed baulk was installed to prevent retraction of the droop lever until the speed was above the stall speed. BEA said that the likelihood of someone retracting the droops before the stall speed was exceeded was very low because the droop lever only became unguarded when the flap lever was moved, it then only took something like 10 secs for the speed to build up to exceed the stall limit.