In the last two days I’ve had two ‘odd’ occurances with OSX, both related to functionality that just ‘goes missing’
Firstly, yesterday I was working in iPhoto when it crashed and I used the ‘restart’ option in the notification popup about the crash to restart iPhoto.
As an aside, at least crashes in OS-X happen faster than in Windows 
Anyway, after restarting iPhoto I found that i could pretty much do anything except drag and drop photos into album’s I had created! It took a quit/restart of iPhoto before the functionality came back (very odd)
I had a related issue today - the ‘hot corners’ on my desktop stopped working - I had to go into preferences and make a small change to the hot corners settings before it started working again.
It makes me thing that both drag and drop in iPhoto and hot corners are some kind of background process - which in both cases had died - rendering the functionality unavailable.
Anyway, it was one of the few oddities I’ve had so far with Mac OS X.