Mostly on high populated/high ping server, but not exclusively.
-Crouch and start to proceed fast pacing
-stop to proceed fast pacing the moment you push the button to stand
-few moments later try to ADS, you should be not able to or with a huge delay of a few seconds.
The stuck can consist in you unable to ADS for some seconds or sometimes you have to push the ADS button repeatedly since you come out from the stuck situation.
I have this problem on an high populated/high ping server (Those video are mostly from DayOne Livonia server) but I have been able to reproduce the problem even on Vanilla official server.
The difference is the reproducibility.
On DayOne (high pop/high ping) I have a reproducibility of at least 50% while on Vanilla I spent couple of hour to have the problem happen only few times.
It can have maybe something to do with loading ''new assent''.
I say so because the only times I could be able to reproduce it on Vanilla server, something new was happening on my screen (passing next to a chicken, passing from an empty zone to a zone packed with zombie, fights scenarios).
So in the equation seems to be involved server load somehow, even without high ping/high population.
The high ping/high population on a server skyrock the problem to umplayability.