> Every build starts from a clean room. You can never get a different result because someone installed something on the builder last week.

At least 6 years ago, this was not entirely true: a package happened to build only because, by happy accident, the builder had a dependency installed by something else. Since the dependency was not in its Build-Requires:, the build would fail in COPR, which really used a clean room. A case of being saved by the alphabetic order, I guess?

Maybe they have updated Koji since.

Mmm, COPR and Koji I think use the same mock configs but someone else would have to confirm that :)

It's definitely possible that one of a packages build dependencies pulls in something that a package /should/ depend on and then when it drops it suddenly the build fails. That's why I think we mostly want 'autogenerated' BuildRequires nowadays that are generated based on the upstream source or packaging format to make that less of a figuring out excercise.