Days of Yore
I've only just come across this thread and thoroughly enjoyed reading some of the tales. This may not be classed as a 'cock-up', as, IIRC, both incidents were a result of technical failure. Nevertheless, the Battle of Britain At Home Day held at RAF Coltishall in 1970 did not go according to plan. A Lightning T4 ended in a smoking hole just outside the MATZ - both pilots ejecting successfully - and a Mk1A engaged the barrier on landing following a brake malfunction.
Coltishall had witnessed a worse incident earlier that year, but the display was not a public one, just a bit of showing off for the AOC which rapidly went to worms.