Laucher fails deleting natives

I was playing, like always, i never close the launcher window. when i closed the game i saw something like that:

Game ended with no troubles detected (exit code 0)

Ok, i closed the game, that what was supposed to happen... But then, i saw something like that:

Deleting C:\Users\MYUSERNAME\AppData\Roaming\.minecraft\versions\13w18c\13w18c-natives-12438325536310

Ok, the game is trying to delete some file, i don't know why, i think that is supposed to happen, but then i saw that:

Couldn't delete C:\Users\MYUSERNAME\AppData\Roaming\.minecraft\versions\13w18c\13w18c-natives-12438325536310 - scheduling for deletion upon exit

I don't think that is supposed to happen, why the game will try to delete a file, if it is supposed to fail?

Sorry for my bad english, i'm from brazil.

i changed my cumputer user name for the word "MYUSERNAME".