And examples I've seen were Kasumi's PP and P6P but I've had both held in and out of stun
Free Step Dodge
Chapstick
Lulu
Well some natural combos only work on normal hit and others work in Critical Stun
Darkslay
Its a string that can't be held, blocked or sidestepped, some NCs will also "jail" on block so you can't hold in between. As Lulu said, some NCs will only work on Counter Hit and some even can be blocked but not held and viceversa.