I just upgraded to Windows 8.1 and when open osu! I get this problem.
Mouse cursor can't controllable when setting Mouse sensitivity higher than 1.0x
I'm trying to rotate my mouse.
At 1.0x It's normal, but when set to 1.1x. It's look weird.
Then set it higher It'll uncontrollable.
Edit: Lower 1.0x have same effect, only 1.0x working fine.
Precision disabled in mouse setting. No external mouse software.