As a Java Programmer for years, I barely use JUnit to test my code, except testing some implementation of challengeable algorithms.
Today, one class I wrote several months ago cost me 3 hours to find the bug in it.
I almost went through every corner of my project to find which could be the cause. Then, I just realized one copy-paste produced mistake is the reason for all. This makes pretty mad, especially these kinds of problems waste the time this long.
I think it's time to start using JUnit to test every piece of code right after I "thought" I finished.