One silly thing in OS X is that if you have only a wireless keyboard and mouse, and accidentally turn off Bluetooth, it is really difficult to figure out what to do next. The usual advice is "attach a wired mouse and keyboard and turn it back on", but if you don't have one, lots of luck.

You can actually recover from this with a full power down and restart, OS X tries to detect this condition and turn Bluetooth back on. But sometimes it doesn't work and requires a PRAM reset.