Sure we might want to date someone who is unlike us, maybe for a challenge, or novelty factor, but there are simply types of person we must avoid dating at all cost.
Married people. This should be obvious enough. A married person is simply bad news. If you are married and want to start an affair, you are risking losing not only your spouse, kids, house and financial stability just for a single day or two a week. And if you're single, you must accept the fact that the person would be highly unlikely to leave their spouses for you. You're dating, but you are for the most part still single. That, and the reality that all the lies and deceit you have to come up with just for the two of you to have some time together are simply not worth giving even the slightest consideration.
Serial daters. This type of people is difficult to recognize, as they use all the appeal and wit at their disposal to make you melt in your toes. They say what you want to hear, and before you knew it, you've already fallen for them. Be careful in falling for this type, as you might be another one of their trophies, good enough to be shown off to their friends for a while but dump you in the end for someone new.
Liars. While everyone tells a lie to try and impress someone, sometimes these white lies turn to something ugly once found out, especially if you've fallen for that person. Be suspicious of any outrageous claims. Try to find out if there's any truth in what you've been told.
Flirts. Yes, they are fun to be with, and you'll never be bored in their company. But there is the fact that they simply cannot help but drawing all the attention to themselves every time you go out. Too much so that you'll find yourself left out as other people flock around them. You must be very secure to date this type.
Rebound person. You will notice if you're just being used as a rebound when the person you are dating keeps referring or mentioning an ex. If this happens, gently let them down and walk away. It's obvious that they aren't quite over their previous lovers yet, and not quite ready to start a new relationship.
If you do find yourself dating one of these types, try to back out gently. You could tell them you had a great time, but doesn't think you're ideally matched. It's better to walk away early on than subject yourself to much pain.