In this particular case the upgrade was interrupted. I have around 250MB on my / partition now. But as I prefer to upgrade from text console, I caught all "out of space" messages and fixed it trivially (made more room, repeat the command). That's the only problem I ever had with a Debian upgrade. I typically use testing.

Now Ubuntu upgrades are hidden behind a GUI, and you are told not to do it manually. It's not uncommon that they fail to cover all difficult cases. Either way, I have Ubuntu on my mother's machine and I've become scared of making upgrades. A few times I had to drop to IRC to ask for specific instructions because I couldn't figure it on my own.