Today's example:
A few years ago the Xserver failed to build on SPARC. @thesamesam fixed it in https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/813. The fix added a necessary bit of code to the build that had been missed in the autotools → meson transition.
Recently we discovered it had stopped compiling at some point when GCC became more strict! (Fix: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1751)