"Here's our <....> module. It's not working, and we need it fixed urgently."Problem is, this module isn't actually part of our product. It's something one of our developers here built specifically for this client, and that guy is currently on holidays. There is no developer documentation, no user documentation - heck, I don't even know what it's supposed to do. However, my boss tells me I need to fix it for them. See, I don't boast a bunch of industry experience, but if I was running a company and one of my developers built a client an undocumented database module, and the client then rang up for support on that module while the original developer was on holidays, I'd apologise to the client that we couldn't help them at the moment, and then fire the developer. Or at least give him a warning. How on earth am I supposed to support something that the client knows more about then I do? It's worth saying that there's no policy in regards to developer documentation at my work, but personally I think it's a bit of a given - especially where you're doing something that's client-specific.
OK, end rant. If you're reading this and you're a developer: DOCUMENT YOUR WORK, BUGNUT!!!!! Some poor schmuck will be supporting it long after you've forgotten you wrote it.