When to write a unit test?

In my opinion the following are situations to implement another unit test.

• Before writing new production code (TDD)
• Before fixing a bug (exposing the bug with the test)
• In order to get to know an API where the documenation doesn't help much
• When encountering untested yet critical code (e.g. using a coverage tool)
• In case you are bored and have nothing else to do ;-)

Of course, when strictly following TDD, the fourth situation cannot occur. However in real life we have to cope with untested legacy code.