I admit that I am 100% guilty of not taking this advice in the past. I met one ex-boyfriend after his married best friend hit on me. Then, there was the guy whose friends all made a group pact that they would never get married.
As a psychologist, I've been asked frequently about red flags or warning signs that may contribute to violent or aggressive behavior. For the most part, dozens of interpersonal red flags can be summarized by two alarms: change and disclosure of information.
I would never get into a relationship with a man who was constantly contradicting himself and constantly changing his views on things based on appeasing other people... and I would never vote for a President like that, either.
High in the skies of euphoric love, if any red flags pop up, you manage not to see them. Once you fall in love, parts of the brain go dormant -- the parts responsible for wariness, suspicion, discrimination.