There's good and bad news on the figure 8 itself. The good news is that they're visually easy to check -- any screw up will make the knot look very different. The bad news is that they invert at relatively low loads. That's mitigated with a long tail.
However, I've never seen any evidence that those single fisherman's people tie in the tail actually do anything. The key is making sure the tail is long enough that the knot is still tied if it inverts.
That's crazy about the rockfall -- lucky you guys weren't standing underneath!
I oversimplified my knots explanation so i wouldn't bore anyone. Isn't the fisherman's knot supposed to jam up the knot if you missed a loop of the figure 8?