When you are first getting to know someone, risky topics are PAST RELATIONSHIPS, religion, politics (they might fall asleep ;) and sex.
These topics can lead to the wrong kind of heated discussions and are sometimes best to avoid, depending on how well things are going.
People who constantly bring up a past relationships are probably the biggest problem here. Your new potential partner really doesn't want to hear about your past failures, even if it wasn't your fault.
See for dating conversation starters you should be using to get the initial conversation flowing.