I'm sorry, I can't let you do that Dave... because you're not Dave

We have a computer. Well, more than one, technically; but only one that will speak to one particular printer. And since that's the only printer that can print posters, when we need posters we need that one computer.

So when that one computer decides not to talk to that one printer, it's a problem.

The weirdest version of this problem, IMO, is that every so often the computer suddenly refuses to log on to our network. (Both the computer and printer are on the network via Mac's Airport, not directly connected to each other.) If you try to get on to the network, or to perform any diagnostics, the computer pops up a little window asking for a username and password. But not just any valid username and password—one specific username and password.

The first time, it required the u/p of my colleague on the other side of the building. She entered the info, and the computer cooperated. For a time. Then, like an addict unable to quit, it came back and demanded it again. Again, she complied. Now it's begun asking for the u/p of one of our IT liaisons who doesn't even work for us anymore, not even in the same building!

Oh, we tried entering other u/p's: mine, my aforementioned colleague's, etc. Nada. It wouldn't even give an error message, just go back to refusing to access the network.

Fortunately, I found at least a temporary solution:

In "System Preferences" —> "Network" —> "Airport", there's an "Advanced" settings button. Clicking it opens a window of tabs with all kinds of esoteric names, including one that reads something like "802.1X". Under that is a list of "User Profiles".

For our particular computer, there was only one "User Profile" listed, with a little check mark in the check box next to it. And in the "User Profile" itself was... the very username and password we were being prompted for!

Unchecking the checkbox next to the "User Profile" did make the computer stop asking for a password. But it did make it accept my own u/p. Hopefully, it will also accept others' when necessary. But even if it doesn't, I'm just in the next room, so that's a bit more convenient. :-)