Sturmey Archer 3-speed slips out of 2nd gear. Goes from engaged to free spinning while pedaling without cable movement. Seems to work alright in 1st and 3rd. It's 1979 and has not been ridden in years (maybe a decade).
1992 Marin Eldridge Grade, 2007 Kona PHD and 199? Trek 1000 (current build project)
+1 - That's what we kids in the 70s and 80s used to call 'slip gear' on our Raleigh Grifters and Choppers - it just needs the gear chain setting up properly which the link dsbrantjr posted tells you how to do.