Back in the 1970s there was a stall incident on a Standard VC10 during a CofA airtest. The Standard VC10 had wing fences on both the wing and the leading edge slats such that when the slats were retracted they abutted to form one continuous fence on each wing. In the clean configuration air leakages outwards along the span were prevented by rubber blocks.
On the flight in question, I can't remember now whether the blocks on one side were missing or merely damaged, however, when they came to do the clean stall the air leakage through the gap was sufficient to make that wing stall before the other. The wing dropped and they went beyond 90degs of bank before recovering. This would have been up at somewhere between 15,000 and 20,000ft.
They recovered the aircraft without damage and it was only afterwards that the missing/damaged blocks were found. After that, one of the checks before a test flight was always to inspect these blocks.
One lives and learns!!