https://i.gyazo.com/47024a8a98d63bdfb013518ea765ed5b.png
It happens quite a lot since recently and seems to be randomly triggered in a lobby, resulting in being "kicked out" of the lobby (quite annoying as an ARAM player)
It means they were unable to start your game, clearly. :P
...In all seriousness, it looks like a general error message, only slightly less vague than "Sorry, something unexpected occurred". It doesn't mean anything by itself, Riot'd have to look at whatever logs they had to figure out what happened.
spaghetti code
