I've had this bug pop up on a few occasions (all but one occurred when the client attempts to close the game window when a game completes - this is mainly on ARAM).
The other one literally happened to me in my last game (this time on Summoner's Rift), where I was in the middle of an early game skirmish, I dodged a thresh hook and boom screen goes black and I cant do anything (with the windows error sound popping up).
When I try to investigate I could bring up my open tabs (alt+tab) but cannot close or switch to one. I then found the tab showing a "Critical Error" message.
I then proceed to try to get out of the frozen hell by opening task manager (it didn't help, much to my chagrin). Instead by viewing my tabs I could see that the client now proceeded to hold an excessive amount of memory.
In the end I just pressed the windows button for a while till the menu popped up and then logged out and back in, before getting into a game where my lane had now lost and I was subjected to toxicity (While it's understandable, it isn't nice to have people become toxic due to something beyond my control).
Also hoping their reports don't result in anything, because I don't want to have to try to gather all the log files to hope I can prove that my client had a "Critical Error".
No offense meant to anyone, be it Riot or anyone else but seriously, every time I begin to enjoy League again something like this pops up and dampens the fun and them I'm sat there not wanting to start a game in case it all goes to hell. Literally ended up almost exclusively playing ARAM because it's far less mentally taxing at this point.