Astoundingly fucked up, yet true. I just got an email from a client who said she'd accidentally deleted a file she had open, and when she closed said document it was gone. I was all set to email her back saying that her file could not have been deleted if it was open, because the Mac OS won't allow that sort of thing, but I thought I'd try it first, just to make sure I wasn't talking out my ass (which is something you learn to do after many years in systems work — try stuff before you speak, that is, not talk out your ass, though that does come with the territory as well I suppose.)
So I tried it. I opened up DVD Studio Pro, created a new document, saved it to the Desktop, and then — with the document still open, mind you — I deleted it. Put it in the trash and deleted it. I received no warning message. And when I quit DVDSP, I was not prompted to re-save the document. It was gone, daddy, gone.
As astoundingly fucked up as this is, I do understand exactly why this happens. You see, active documents — that is, documents which are currently open — are stored in a temporary location, not, as one might suspect, where they are saved. The saved document and the open document are two separate files. Each application determines where the temporary storage location for open files will be. In the case of Final Cut docs, for example (and I know this from cold, hard experience), the temporary storage location is in a folder called .TemporaryItems on whatever drive is the active working drive. So, since the saved file and the active file are actually separate, it's very possible to apparently delete a file that's open in an application, because the file you're deleting is not the open file you're currently working on. It's a saved version.
I don't know if I mentioned that this is fucked up, but it really is. Astoundingly so.
In the same way that you can't delete running processes, I think Apple really needs to make it equally impossible (or at least as difficult) to delete active files. The current paradigm just doesn't make any kind of sense from where I sit.
A reader has pointed out in the comments that one application actually works the way you might expect. That application is Microsoft Word. A saved Word document and an open Word document are (apparently) one-and-the-same, and Word is able to keep track of open files even when they're moved. If the fact that Word behaves more sensibly than most Apple applications isn't an indication that this is something that needs fixin', I'm not sure what is.