BADMOUTH THEIR EX OR EXES. If you're on a date (or even before on the phone or texting) and they talk badly about an ex or the other parent of their child, this is a HUGE no-no and in my opinion a deal-breaker. Now, if they are coming out of a tumultuous or volatile relationship, and you ask them to share- this is much different.
RUDE TO THE WAITSTAFF. If they snap once because the food is ice cold, then maybe? But if this person is consistently rude to bartenders and waitstaff kiss them goodbye. Buh-bye!
CRICKETING. If they take hours (or days) to return a text or phone call this is called cricketing. It's like when you "hear crickets" ya know? You text...and nothing. You call...and nada. But when they do finally call back or text it's like nothing's wrong, and you have a great conversation. Rude. Disrespectful. And if I'm being honest mean. Not someone you want to get with long-term. It's just going to frustrate you.
SORRY, NOT SORRY. They never apologize for anything. Never. Oh, and they never accept blame for anything either. RED FLAG. If someone can't take ownership or hold themselves accountable for what they've done wrong in a relationship you need to RUN. There are bigger fish to fry there.
LIE LIKE A RUG. If they lie and fib once or twice, I can guarantee they will lie three or four times. Do they exaggerate, too? Like lots? Or is it just fibbing to make a story better? Lies are like poison, y'all. Just a little bit can hurt ya. And if they don't tell you something, and you find out later...that is lying by omission and it's STILL lying. I hate that shit.