Sometimes 2 people will contact me, asking me to do a direct swap between them. Something they've discussed and arranged for whatever reason. I see no problem with that, as it doesn't effect anyone else. As this has happened a couple times, and I've come to realize it's a good tool and (I think) perfectly fair, I've offered it a couple times to good peeps in need.
Usually, when there's a dropout, I just move down the list, in order, asking, "are you able to go early, or do you want to wait your turn?" *
If there's a really last minute dropout, something that's going upset my schedule, and is frustrating, and will basically cost me money in work time lost or fewer mods produced, I improvise. I might shoot for the next person on the list who I know is actually ready to roll. I might just make a random mod and see who wants it when I'm done. Or I might feel it's an opportunity to insert some chaos of my choosing into the order and fairness that I put all those hours into creating and maintaining. I feel on those rare occasions it's my prerogative.
*currently, due in large part to this, and in some part to increased productivity, I'm doing the fifth actual run of mods; but am half way through "run7" on the list.
no more hate for you, only jealousy now 
then check bzck later.
