Their Political and Religious Beliefs
People's political and religious beliefs are too closely tied to their identities to be discussed without incident at work. Disagreeing with someone else's views can quickly alter their otherwise strong perception of you. Confronting someone's core values is one of the most insulting things you can do.
Granted, different people treat politics and religion differently, but asserting your values can alienate some people as quickly as it intrigues others. Even bringing up a hot-button world event without asserting a strong opinion can lead to conflict.
People build their lives around their ideals and beliefs, and giving them your two cents is risky. Be willing to listen to others without inputting anything on your end because all it takes is a disapproving look to start a conflict. Political opinions and religious beliefs are so deeply ingrained in people, that challenging their views is more likely to get you judged than to change their mind.