The only answer that anyone should give is: "ALL" of them. A good operator should take every mistake serious. An exceptional forklift driver, will understand that accidents whether minimal or serious does happen. I've been certified numerous times, through different companies. The worst mistake I've ever made though, was bringing a pallet down, and backing up and into a radial arm saw. I did disclose that the forklift brakes were gone. Even though theoretically wasn't my fault, it bothered me