Signs Your Relationship Is On The rocks

The interview was very popular so I have decided to use the questions they asked along with my answers in a series on relationships. Here is the next question.

At which stage does one notice that they are not in a healthy relationship and which steps should they take? Relationships are complicated and never a one size fits all answer, but there are a few signs that I think are pretty universal to any relationship which is moving away from a healthy relationship towards unhealthiness.