Casting a targeted ability as a target dies can sometimes leave it on cooldown clientside

Even though the server reports it as unused.

Example: Using Darius ult (noxian guillotine) on a target as it dies (so the ability remains unused) can sometimes render the ability unusable on my end, while if I ping my own ability for everyone to see, they see it as ready. But since the client thinks it was used, it remains locked out for the entirety of the cooldown duration.

Largest offenders for me: Darius ult, Vlad Q

Also know can happen from experience: Morde and Garen ultis