I sort of fell into it. An engaged friend of mine initiated a relationship with me. It turned out to be fairly unhealthy and ended pretty miserably. Some red flags...
Not properly communicating desires/limits with each other
Placing excessive rules on outside relationships
Attempting to isolate a partner
People not willing to meet you halfway
Partners who give you none or very little power in the decisionmaking of your relationship
Metamours who don't welcome you
metamours who refuse to work through their own problems
People not wanting to give your relationship a label even after being together for months
Highly closeted folks
There are plenty more. Not all apply equally to everyone. There are plenty of good things too. Just tread carefully.