If you have shutdown problems on your Mac: it never shutdown and is stack at on the grey circular icon. The problem could be your TimeMachine setting.

I just discovered that my MacBook had problems connecting to the network timemachine disk (it's not the time capsule but a Buffalo NAS)... No backup were executed, sometimes the system was unusable and cannot shutdown or restart (need to force the shutdown with the power button). Well i don't exactly know the reasons around the missing connection to the disk, but after a reset of my time machine settings, all back to normal.

So if you have some strange problems in your Mac, check your latest backup and try to force a new one by hand; if it cannot find/connect to your disk... you got it! :)