A long time ago, I did a project where I downloaded a year's worth of nightly builds for Thunderbird so that I could collect nightly code coverage information. Over the course of doing so, I discovered that there was one dependency (pango, I think?) such that no version could support the entire year's worth of source--the newer version didn't work with the older builds, and the older version didn't work with the newer builds.

Come to think of it, in terms of trying to get old code building, the CVS days of Firefox should be interesting... because the first command in that build step is "download the source code" and that CVS server isn't running anymore. And some of the components are downloaded from a CVS tag rather than trunk, and the converted CVS repositories I'm aware of all only converted the trunk and none of the branches or tags.