I'm not going to edit your quote, as it is against the rules, but if I may paraphrase your reply below:
simple hold on tight, I've been on some river rapids rides, and I've never fallen out. I game enough to put money on the fact that he responsible. Thousands of people would ride those rapids every year. If it were a design issue one would expect people to fall out of Thunder River Rapids more often. Not just now.
Nobody realises an issue until an issue occurs. I'm not saying you're wrong about it being rider error, i'm just saying that your line of thinking "i've never fallen out so it must be his fault" is flawed - you could just be really lucky.
With that said, I feel as though these guys may have been goofing around.