If there are stale tracks in the current playlist, they are cleanly skipped. 
however, they are still counted and are still hidden in the backend list.  There
are two side effects to this. 

1) in fishbone skin, where is includes itemnumber in the list, you can see the
missing numbers.  On one hand, that might not be too bad since it does reflect
stale data. 

2) however, say tracks 1-3 are stale.  The current playlist will show track 1 in
teh list anyway, and if you delete it you then see track 2 pop up out of nowhere
in its place.  This is definitely confusing.  

I suspect that the best way to deal with this fully is to simply dump any stale
track from the current playlist, either as it is loaded or otherwise
encountered.  Proper logging can keep track of those references in case users
need to know.