In 2016 I joined C project initially created in 1999. This project had ~7k lines in gitignore and I learned this only because one of my commit created 2h of email exchange why my code does not compile. Turns out filename was forbidden by one rule.

This one file was more sophisticated than any other file in the project.

It contained "good practices", editors/IDE files of applications dead for more than 10y, personal. /tmp directories in various names, files versioning using suffixes and comments for sections v of rules starting about in half of the file.

This project learned my to keep your own shit in local global gitignore not in project.