Popped by here today and saw this thread, as a club figure myself it's the sort of thing that I address a lot.
The easiest thing is to not make the mistake in the first place, but that obviously isn't always plausible. What our players do much of the time, is "give it to the other guy" -- our term for accepting penance for our own mistake. So when a dice roll like the one in this thread comes up, many of our players will go ahead and say that the least favorable possible way the roll went is what happened, so to make things fair and not appear to be gaming for a free re-roll.
In response the other guy usually does the good guy gamer thing and lets the person re-roll it anyway.
The spirit of how the error is addressed, in my experience, is more important than anything.
I have heard of another interesting method at resolving this thing: simply apply the dice as if they were rolled one at a time, and consider the one the furthest left to be the first roll and work your way to the right from there.
So, for example, if the closest model that would be taking the initial saves was a marine, and you rolled 1, 2, and 5, this guy would look at where the dice physically landed and apply their results in order from the leftmost die to the rightmost die, for better or worse (i.e., the 1 applies first, killing the 3+ save model, then the 2 and 5 on the terminator).
This is a good objective way to deal with it, basically just pretend they were rolled one at a time counting the leftmost die first. We've never had to use this because of my club's general "My bad" attitude, but it has been useful for me in tournaments occasionally.