I was type training an experienced pilot on an amphibian yesterday, and came closer than I would have liked. I have trained him, as I do, that EVERY landing is preceded by speaking aloud, while verifying gear position is as required. I told him that he was on a 3 mile base to a water landing, and directing him toward the desired area. As it was to be his first glassy water landing, there was a bit of extra discussion. I told him to configure the plane for a glassing water landing. I was also focused on where we would land, for the greatest safety and training benefit.
While doing my independent "save butt, save face" check, I noticed that the wheels were still down. He was turning final at about 500 feet, and pulling power back. He was reading through the very brief checklist, but not actually touching or otherwise confirming what he was reading off the checklist. It was the shopping list, and he was just going in the door of the store.
I waited a few seconds for the best time (still 300 feet up), and calmly said "what about the gear?" then, before he had tome to do anything, called a go around. He did!
Between us, I expect that our total flying time exceeds 12,000 hours, with half of that is RG aircraft. We came closer that I liked to getting it really wrong. Had I solely relied on him, I'd be wet, and being written about on the internet.
Checking the gear has to be instinctive, and actually happen. It's configuration assurance, and it is a vital pilot skill and discipline.