Yesterday I noticed a one liner which included an unintuitive chaining of calls. I asked my colleague about it and we briefly jumped around the code and saw a passing functional test which supposedly indicated that all was well, and moved on.
A couple of hours later I decided to try out the application through a web browser and observed that the application would fail on any request.
I went back to the questionable code and realised that the class that contained it was not covered by any unit tests.
Half an hour or so later the component in question had its own unit tests and an additional half dozen lines of code to make it perform its intended purpose.
As with many things in life, with the benefit of hindsight I realise that I should have paid more attention to when my spidey sense told me our use of the API didn't look right.