Today, as an evening amusement, I fixed a very nast Bug in the #Koha test suite, that did not occure when running only the affected test file, but only when also running some other tests: https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=39700
The root cause was a function returning a list, but the test calling the function in scalar context. Unfortunately a classic #Perl footgun :-(