I was in a situation last year at my sisters wedding where the pro paid 'tog simply didnt show up (turned out they had a serious accident on the way) and I was asked the ominous question... 'will you do it please big brother?, you have the best camera here and the only one with a clue what your doing'. Pressure? bloody right! no recce, military honours wedding at St Marys Chapel in Edinburgh Castle, Easter Monday and teaming with tourists... brown trouser moment was one way of putting it.
I had the non VC version of the Tamron 17-50 f2.8 and my 70-200 f4L with me anyway to take my own shots so wasnt too badly off for kit. Thing is, at that point I had no interest in shooting people in any situation, never mind weddings (still dont really) and it was a very very quick bit of research on my phone for required shots and pretty much wing it from there. All went much better than I expected, got the shots I wanted and what my sister and her new hubby wanted.... then disaster.
I was taking shots of the evening reception, people wishing the happy couple all the best at the bar, people dancing etc. I sat down to get a drink myself and went to look at the images I had just taken. Hit the review button - showing image 1/1. OH MY GOOD S*%T! yep all images gone. Not a clue how it happened, camera wasnt out of my hand at all, only thing I can think of was I lifted the camera up to let a couple of people passed me and somehow I may have hit the menu button and it was on the Format function. I tried and tried since but cant recreate it, but its the only logical explanation I can come up with.
Anyways, I shot up to our hotel room, laptop on and tried to transfer images from the camera via a USB cable as I had no card reader with me. Nothing, just the 1 image the camera showed me. By luck I had read about recovery programmes in a camera mag, I got online and bought one but had to wait until next morning to go out and buy a card reader from the local tesco and hope for the best. So with nothing else to do, trying to look as unsubdued as possible I went back downstairs, explained it quietly to the wife and the both of us killed a bottle of Bushmills whiskey and a bottle of apple sours before going up to bed.
First thing next morning straight to Tesco and got a reader, lucky enough all images came back ok and I got an album put together for them.
Sorry for the 'cool story bro' type content but just wanted to show how easily one silly mistake or fault can cause a major problem. If you feel happy doing it, crack on, everyone has to start somewhere, but as said, do your research well, get your gear sorted well in advance, back up everything, learn how to be a diplomat, comedian and pre school teacher (trust me, when the drinks in the guests, you will see what I mean

) and once your sorted, have fun.