Unfortunately there's no box or error message that pops up informing us that we've scheduled someone outside their availability or on a day that they are unavailable. When doing the schedule, we click on a day of the week next to someone's name, and a little box pops up that shows, among other things, what times that person is available that day (or if they are unavailable that day). On that box, this info is below the options to add a shift for that person for that day, delete them from that day, edit that day, and mark them for swap. But those options are the important ones that the schedule writer is looking for. So if they are working at a computer with a small-ish screen, then the availability for that day will actually be out of the screens viewable area, and they might not scroll down because the thing they want to do, (add, remove, edit, swap) is showing.
This assumes the availability is even showing correctly. Needless to say, the system is pretty crappy (we can't swap two shifts for the same person on the same week? Come on, really?).