When and how do you find out that your private method doesn't handle an edge case? It seems like out of a matter of practicality that sometimes it would be bes

When and how do you find out that your private method doesn't handle an edge case? It seems like out of a matter of practicality that sometimes it would be best to have a unit test for a private method- dogma be damned. Personally, if it's tricky code or something I worry about, I test it. If you change it, write your own damn test :-)