This isn't twitter specific, but from what I have seen, things can fail for the following reasons. Something that autoupgrades would become out of spec, refuse

This isn't twitter specific, but from what I have seen, things can fail for the following reasons. Something that autoupgrades would become out of spec, refuse to start due to a change in config files. A counter would roll over somewhere. A disk fills up. A cert expires. A domain name entry expires. Could be days/weeks/months.