I haven't seen to many failed RCDs so far, and most failed on the safe side - tripped and couldn't be reset any more. Only one or two refused to trip, on one even the test resistor blew up spectacularly.
Same goes for MCBs, but I did encounter one very old range (Schrack HLS, manufacturing date stamp 1960) that refused to trip as a whole. There were a few dead shorts (faulty light socket) but the MCBs (10A H curve) never tripped before the 20A gL main fuse blew.
