What are the common reasons for condom failure?

I live in Prague, Czech Republic — Europe. Before asking any question, I should tell you that I really appreciate your service. As for my question, I would like to ask you about the most common reasons for condom failure as a contraceptive method. I suppose that the most frequent one is condom breakage. But what are the others? I am especially concerned about pregnancy risk implied by the accidental transfer of sperm in pre-cum to condom by fingers while putting it on.