League on Mac
This may be a mac exclusive bug, but sometimes I would get "An Unexpected Error has Occurred" and miss the champ select ready check.
Secondly, while selecting roles in draft/ranked, normally a GUI pops up with the 5 roles. Sometimes this GUI would appear at the bottom of the client window, where it shouldn't be.
There is also huge lag from having a large friend list, and that usually slows my outside of game league client to 1 frame per 5 seconds. The solution is to collapse the friend list, but that may take up to a minute because of the lag.