In the days when a PDP 11 was considered a powerhouse and was running a
particularly important system, our box was suffering an intermittent fault causing
random lockups. The engineers were baffled, running test after test and after a
couple of days it was decided to strip the box down in situ. The top was removed
and the cause of the problem immediately became clear: the small "Passed Quality
Control" sticker's adhesive attached to a vertically-mounted card had dried out and
the sticker had lodged itself between an interface card and mainboard slot causing
loss of contact.. problem solved.