A foot long piece of wood was discovered in a Buccaneer which suffered a roll restriction at low level - and one of the early Tucanos was delivered with a workman's left over lunch in the back. Such was the build quality of the early Tucano that after they'd sat outside for a few days (to make sure that no Irish terrorist had left a little gift), they were all de-panelled and given thorough checks...
A VC10K2 in which we were doing an air test suffered smoke on the flight deck, so we did the relevant drill and landed. They found a rivet head in one of the cabin conditioning blowers, but weren't sure whether this was due to sabotage or error - the RAF was about to hand over to civilian-run majors at St Athan and feelings were high. So a complete search was ordered - but the only thing of note (apart from the fact that the lift rate modifier had never been connected to the stall warning system) was a BOAC silver teaspoon found nestling around the base of one of the control columns! It must have been there for years - this was a Standard VC10 which had been sold to Gulf Air by BOAC and had flown many years with them before coming home for conversion!
But Tengah Type gets the prize for the Sunderland dining room chair story!