I saw this in Perl a long time ago but it could just as easily have happened in Python. The dict (Perl hash) was a set of mappings for template replacement of "from" strings to "to" strings.

    "FOO" => "bar" # Replace FOO with bar

The author had considered the case where one key (FOO) might be a left-substring of another (FOOBAR), and so reversed the output from keys() before iterating over the hash. This ensured that FOOBAR, which sorts later, would be substituted before FOO (else FOOBAR -> barBAR, oops).

Unfortunately, despite empirical evidence to the contrary at the time, keys() does not guarantee sort-order results https://perldoc.perl.org/functions/keys.html

This worked for a while (appeared to work?) and but was subsequently caught by another developer, who added a sort().

(Get a templating library alreay yeesh).

Incidentally Ruby made the transition to sorted Hash some years ago (1.9 maybe?) and I don't remember any great fall-out.