Not sure if this is the reason, but at my job, if you need to get someone to work for you or switch shifts, once you find someone, you have to call the GM, and the person working for you also has to call, and just let the GM know, yes, I'm working for so and so on this date. This way, everyone knows who is now responsible for that time, and if the replacement fails to show up, the original employee isn't on the hoook
This is how I handle schedule trades with my area. Sometimes, someone new to the system (someone new to me handling his/her schedule or newly hired) will try some stunt in "authorizing" his her own trade but is set straight pretty quickly (even our dept director, the top dog, has to have his out-of-office time approved by his supervisor).
We had a schedule trade mess-up a few months ago when a new hire agreed on a Friday afternoon to stay an additional hour Sunday afternoon to cover the start of the shift for someone who needed to come in late. Well, the new hire forgot he agreed to stay and left. Also, the 2nd employee had gotten the change approved after I was gone for the weekend so the schedule change was not noted on the posted schedule. Mayhem ensued for the hour that they were short-staffed, culminating with the moronic supervisor in charge that weekend sending an accusatory e-mail to all other supervisors and me blaming me for the schedule not being up to date. Funny, I didn't get an acknowledgment or apology from her after pointing out on the e-mail notification from 2nd employee that the approved change came AFTER I was gone for the weekend (I don't think I needed to point out to everyone what an unreasonable idiot she was for flying off the handle at me like that so I hope she felt really really REALLY stupid for that because her e-mail was that nasty).
Anyway, by making the supervisors in charge of noting the schedule changes, it takes the employees off the hook (so 2nd employee was not at fault -- however, 1st employee WAS at fault and was told if this happens a 3rd time, because this wasn't the first time he screwed up by forgetting a schedule change, he would no longer be permitted to do schedule trades).