I fought with my ex-fiancee constantly. It was never about resolution - always about 'winning'.
Our relationship started as a fun "we know each other through a common friend" sort of thing. It was great - at 25, I was loving her youthfulness and her desire to be fun and evade the boringness of being an adult that many of our peers seemingly assigned themselves to without regard.
Over time, I wanted to move forward and progress my life towards, arguably, the aforementioned "boring life". That thought process was shot down and I just didn't realize I was becoming a part of somebody else's plot to 'stay 19'.
I thought about kids, wasn't sure. That notion was killed before I could form an opinion. I thought "hey, this ex-boyfriend's teddy-bear is a bit strange on our bed", 3 days of yelling to 'let it go'. Was bothered by things like "hey, that boss of yours that you were having a happy hour for 6 hours and he drove you 40 minutes out of the way (twice...while drunk) to get you back to nearby is a bit off. Maybe an unwise move" only to be yelled at for being 'insecure'. Got punched in the face at her friend's wedding but had to lie about it and wait a couple of days to get something that resembled an apology because I "probably deserved it".
We were engaged and she bailed. Typing that all out like that, for the first time, is tough because...UGH WHAT WAS I THINKING?!?!
It's easy to take an outsider's perspective and say "well, you're lucky" or "you dodged a bullet" but I certainly wasn't without my silly and misguided expressions of anger - I twice punched a wall/Ikea furniture (which I immediately repaired while apologizing profusely and feeling the utmost shame). I said some impolite things that I certainly wish I could take back.
I guess the red flags I missed were the fact that, while we all certainly screw up - the good folks apologize. The good couples I know attempt to fix things for the (I assume this is a couple-centric post) good of "us".
I never saw that mindset in my ex. In 4 years, I never saw an attempt at accommodating a habit that may be a hinderance on the 2 due to the stubbornness of 1. That's the saddest part about a broken relationship, at least it is for me.
The biggest red flag, in my opinion, is not identifying that you're with somebody who doesn't appreciate you. That red flag, by the way, is one you're waving...figure it out and fix it!