actually there are some strings that do that, I know pai and rachel have strings like that, where you set the CPU to guard and crouch but for some reason the high part of the string hits them despite the CPU being set to crouch, it's like their ex-highs or something
Pai has 3kp that does this. she also has 3ppp which does a weird thing where if you get hit with the first part of a string which is a low, you can't crouch the 2 high follow ups. I know rachel has one of these but i can't find it atm, i'll have to do more testing when i get home
Pai's 3KP jails on block is a natural combo I believe since it jails (Leon's 66PP etc). Similar to Kasumi's 66PK where you can't duck the high in certain distances becoming a natural combo however this is different. You are getting hit from a high the moment you crouch despite his not being a natural combo. You can hold this move, the others you can't because they jail on block/NH.
Yeah this one (3PPP) jails. Not the ones that jail on block like most other characters. Ones where it's simply not a natural combo but it still hits a crouching opponent from a high.
For crouching? you can even duck high unholdables since it's instantaneous after a setup. A move that's not a natural combo on block however, you should be able to duck and avoid the high right away.