Mostly what will happen to me is I'll have an older project that was saved back in the dreaded global audio folder days. So the audio is long gone and therefore the track(s) is (are) blank. 
The other one I get is from when I used to reord at 48 and open in 44.1 the audio is greyed out, these are almost always just bass parts which are easily re done being they are only backing track files. Whole songs I always finish and rarley re vist. 
I have Sonar back up files of everything so if it was real important I can always find the missing audio.