1. LYING TO AND HIDING THINGS FROM YOUR PARTNER
Even if you're doing it out of love to protect their feelings, keeping little things from someone you're dating can grow into a big problem and cause trust issues that wreck a relationship. As canada432 wrote, "You should be able to tell your partner pretty much anything. If you're hiding something because it would hurt them, then you probably shouldn't do that thing in the first place."
2. WITHHOLDING ANY KIND OF AFFECTION
Whether affection means being really touchy-feely, asking each other intense questions about meaningful things, or helping each other through obstacles, being absent and unaffectionate can cause the kind of doubts in a relationship that end up being irreparable.
3. HARBORING QUIET RESENTMENT
As VoxMeretricis wrote, "by the time resentment is expressed, the damage is already done" to the relationship. It starts out quietly but becomes something big over time, which is what makes it so damaging.
4. LACK OF COMMUNICATION
This doesn't mean not texting all day every day, but failing to talk to each other about things that bother you when they come up, or conveniently leaving out details that you think might cause problems. All that stuff just gets bottled up or revealed eventually, and by the time that happens, it's usually unmanageable.
. BEING STUBBORN ABOUT THINGS OR GETTING ENTRENCHED IN CERTAIN POSITIONS
This is just another way of saying "refusing to compromise." If you like somebody, you should be willing to compromise. Partners who can only handle things if they go a certain way (their way) are basically just in relationships with themselves.
6. BICKERING ABOUT MUNDANE, DAILY ISSUES AND CHORES
Sure, the argument you always have about whose turn it is to take the trash out seems like no big deal now, but that's the sort of thing that just becomes a great issue over time, and even worse, becomes ammo for bigger, more serious arguments down the road.
Talking down to a partner is just another way of making yourself bigger or more powerful than them, and a power imbalance is the last thing you want in a lasting, healthy relationship. As messedfrombirth wrote, condescension is worse than cheating because "it makes your self-esteem terrible," so even after the relationship ends, you still feel the damage.
8. STAYING IN A RELATIONSHIP OUT OF CONVENIENCE
Whether it's because you don't want to disappoint your families with a breakup, or because you have concert tickets in six months or whatever, staying together just because you feel you should only leads to bitter resentment and an inordinate amount of fighting and heartbreak, when there was a chance you might've been able to remain friendly.
This is tricky because it's often so subtle, and you don't realize you're being manipulated by your partner (or doing the manipulating) until it's way too late. As reallybigleg wrote, "the relationships that have left me 'broken' (both of which I left, by the way, so this isn't a case of heartbreak...) have been through gradual loss of self-esteem during the relationship through a process of manipulation by the other partner."
Even without actual cheating, just the suspicion that it's always happening can be much, much worse.
11. PRESENTING A FALSE VERSION OF YOURSELF AT THE BEGINNING
This can be as simple and small as pretending to like horror movies when you actually hate them, or as big as saying you're not looking for anything serious when, in fact, you are. It's best to be upfront from the get to, because those little things can become huge reasons to breakup over time.
12. STAYING TOGETHER BECAUSE YOU'VE BECOME CODEPENDENT
"You're together because you're codependent and neither of you wants to be single," wrote beaverteeth92. Or in other words, you don't have chemistry anymore, and the only reason you're together is so you don't have to be alone.