monline

During a Debian upgrade you'll be prompted about all files that Debian tries to replace, but detects as modified/customized. You get the chance to look through the differences between the old file and the new one and you can choose what to do (keep the old or overwrite with the new ... or cancel the package upgrade alltogether). If you decide to keep the old file, the new one will be placed besides it. If you decide to overwrite with the new, the old one is left there and it's renamed to something '*-old'. The following command lists all files that indicate that a package was upgraded and a config file replacement decision was made:

P.S.: obviously such files can occur anywhere on your system, but for a "well maintained" Debian server looking through /etc should be enough.