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PC - Permanent Feral Frenzy
This has happened to me a long time ago (maybe a year? so well before Legion's latest rework). The only difference between now and then is I didn't see the frenzy animation and I might not have seen survivor's gaining deep wound either. During this you are unable to vault pallets or windows, you are unable to break pallets, you are unable to enter/exit frenzy, and every time you hit a survivor they will appear to get downed but quickly teleport to where the survivor actually is as you hit them with a frenzy stab and not a basic m1. You do not gain a speed boost for this and I don't think you get the chained down either.
My best guess for when this happens is something to do with trying to enter frenzy and vault the pallet as you get blinded, perhaps I get blinded locally but not on the server and end up in a weird state where my client and server info is mismatched so there is nothing to break me out of the frenzy. I have no idea how to reproduce it besides just trying to enter frenzy and vault while getting blinded and it has only happened to me one other time. Fortunately I was able to get blinded back out of it.