I shot my first wedding recently, for a friend's daughter. I vehemently expressed that I am not a professional photographer, and weddings should definitely be shot by a professional!
I am comfortable shooting sports pictures, natural outside photo shoots and things of that nature. But a wedding... my heart still skips a beat just thinking about it.
Obviously I was very nervous, for you only have one shot to get it right. You don't get a do-over when shooting a wedding, you know.