I've got my mouse set so that a click of the wheel acts as the back button. Have done for years over several PCs, and it was working fine on this one (Windows 8.1). Until recently, a click brings up the default scroll bar. So I open the Keyboard and Mouse settings assuming that it's forgotten my setting, to find it's set correctly and now middle click works as expected.
Until I reboot the PC upon which middle click back doesn't work until I open the control centre again.
It's hardly a massive problem but it's irksome. Inordinately so.