LoL Game Client won't launch

While i was queueing for a ranked game i played an other game. I got in the lobby selected my champion and then continued playing the other game while waiting for the other players to pick their champins. When the battle started i still didn't stop my other game and i got a error from lol (i don't remember what it said but it was a DirectX error) i was just wondering if that was a bug. P.S The other game i was playing was Osu in compatibility mode (aka DirectX Fallback) because OpenGL doesn't like my gpu and there is too much lag.