People who had a long term relationship (marriage or bf/gf of several years) fail miserably, what was the most unexpected red flag you missed early on?

I'm not talking about classic red flags--what I'm asking is this: for those of you who had a relationship end very badly (i.e. divorce, infidelity, abuse, abandonment), what was something that, looking back, should have been a sign to you from the beginning that things would not work out well? Something that may not have been glaring and obvious (i.e. rude to the waiter, poor boundaries) and may even be a positive in a different context, or completely neutral, but if you had understood the implications of it, you might have realized beforehand that the relationship wouldn't work?