A C++ project I inherited had some binaries checked in for rarely-updated utilities - no sense in forcing everyone to build everything from scratch every time, right? Unfortunately, this wasn't done by CI, but by hand - so while those binaries could be built on the original maintainer's machine, some stale & missing source files (which said maintainer forgot to `p4 add`, `p4 reconcile`, or perhaps submit?) meant the only buildable copy of source was on their workstation, without anybody noticing.

...which was then presumably lost/reformatted some time after said dev left (quit? laid off? fired?)

Unable to build said source code, a subsequent developer added a python script to postprocess the output of said binary for their own needs. A hack, but it worked.

Then I got my hands on it. Well, I needed data that was discarded by the binary, so hacking on the python script was a non-starter. Instead, I reverse engineered what the binary was doing by stubbing out enough of the source code to get it building - then compared hex dumps of the outputs and tweaked said source code until my modified source code gave the same output as the checked-in binaries. Then I got rid of the python script by moving the logic into the binary's source code, where it really belonged. Then I made my own changes.

Didn't bother setting up CI though. Was porting a project in a heavily compressed (<1 month!) timeframe, and nobody else needed to touch said binary, and the codebase wasn't going to be used for any future projects (it was pretty terrible, and much time had been spent creating a much better codebase for future projects.) I at least made sure I checked everything in, though!