I have recently run into a part that I had never seen fail before other than breakage and I have been fixing stuff for over 63 years, those tubular ceramic caps where the ends are silvered, wire is wrapped around and soldered, and the gap between the silver sections sets the capacitance.  So far it has only been the small low value caps, 30pf or less.  Just had one take out a plate resistor of a SX100 RF amp stage when it changed from 24pf to 519 ohms to ground via a coil.   I know the silver micas go bad, but these recent failures make me more aware to check this style cap now too.  Seems like the giveaway is if you see what looks like corrosion migrating on the ceramic.  So far, the painted ones and the larger value on bigger ceramic tubes have not shown similar failures.

Charlie, W4MEC in NC,