1. Day of or last minute cancellations - I take this on a case by case basis. Sometimes people truly do have something come up that makes it impossible for them to meet you for a date. In general, though, I take it as a bad sign. First dates are about making a good impression. Someone who cancels doesn't appear at all concerned with whether or not you find them rude or thoughtless. Red Flag.
2. They don't send a text to let you know they'll be late - Yeah, dating in NYC and Boston means combating traffic and subway delays, but wi-fi is just about everywhere now, even on mass transit. A five minute delay is one thing. I don't think anyone expects their date to be right on time. Ten or minutes? Send that text. It's rude not to. Red Flag
3. They try to get you to cancel other plans - Entitlement isn't a good look on anyone. Certainly not a first date whom you've never met in person. Nobody should be that aggressive or invested before the first date. Often, it's a test used by narcissists and other kinds of abusers to see just how easily manipulated someone will be.
4. They overshare - Sometimes our nerves get the best of us on a first date. It's not awful if someone tells you about that time they vomited at a work function. But if those stories keep coming or they reveal too much sensitive information about themselves or others, watch out. That's a sign of bad boundaries. Red Flag.
5. They're overly critical - They don't like this person, they hate that show, they're drink is watery. Yikes. Glass half-empty people are usually very difficult to please. Red Flag.
6. They send too many messages in a short period of time - This is a personal pet peeve of mine. Give someone at least a day to respond to a text or email. If you send a follow up message a few hours later or more than one in a day without a response, you'll set off all kinds of warning bells. It will make you look like you have no life, are overly-invested, are entitled, or possibly even unstable.