My central mail exchange and web server box turned up its toes last night and developed a bunch of irritating r/w errors on the un-raided system disk.
So, I pulled it out and put a new drive in (I have plans to move it onto a VM sometime soon, as soon as I finish building the chassis that the VM server will run on).
Now, this installation is very old – it has had its hardware upgraded underneath it multiple times with no change to the OS except for the odd dist-upgrade. So the code base is kinda crusty.
Because of this I decided that it’d be smart to do a completely clean debian etch install, and migrate the data over to the new OS and apps. 
This wouldn’t, I thought to myself, take long.
Wrong.
When you do this you suddenly discover a whole lot of farty annoying things like:
– PHP5 is different from PHP4
– Apache2 is different from Apache1.x
– Databases cannot be physically bitwise migrated between some MySQL versions – you need to do an SQL dump and re-import to make it work.
– Config files change a lot between versions of all kinds of software
..and you also discover that over the years, you may well have installed a LOT of fiddly little scripts and bits of code that do various things that are quite invisible enough to fall off your radar .. until they go away.
Pain. In. The. Ass.