I know its not all ways possible but this is a classic example of when you have a good digital tech on a shoot it can save tyou time and money. For example I have a ST-E2 in my kit that allows you to use AF in almost darkness reliably. check out this article http://www.luminous-landscape.com/reviews/st-e2.shtml
I was hoping someone would bring this up. I've decided to keep my STE2 just for that reason alone. However, won't a flash gun, such as the 430 or 550/580 do the same thing--and you can turn the flash off or down so low it won't show.
One good thing about melt downs or near melt downs is that you learn during or after wards to work through it, including how to improvise on the fly--w/o broken or other technical problems flustering you. But the catch is you have to have melt downs.
Last year I had my new 1DSMKIII go through its "cannot talk to lens" breakdown in the middle of a shoot. I didn't have a spare camera and was 1000 miles from home. What I did was take the lens off, snap it back on, focus shoot, wash repeat for EVERY shot (This particular bug allowed on shot every time you connected the lens.) Shots came out great, and when things like this happen now, if there is a way to shoot, I'll find it
Like others said though. In some cases, you just are not going to get it right. In those instances, there is NOTHING you can do except explain to your client(s) what has happened, own up to any wrong doing, if any, and just tell them you want to do it again at no cost. I mean, what else can you do, except get it right every single time, which isn't going to happen.