Forgetting to lower the gear for landing is another well worn trap. In another era I was training a new co-pilot on the HS 748. The HS 748 had a nifty ASI which, if the gear was not down by I think around 110 knots with landing flap selected, a flag would flash on the dial of the ASI.
We were downwind with zero thrust set on one engine in preparation for a single engine landing. The U/C horn had been silenced because it had sounded with one throttle was nearly closed (zero thrust) and first stage flap selected. It was a long time ago, but I think that was the reason. I was fully aware of the risk involved of inadvertent landing with gear up in such circumstances and kept a close watch on the co-pilot.
He was distracted by another aircraft on long final and elected to extend downwind for separation and wisely elected to delay gear extension since we were on one engine. . The scene was being set.
Now on final at four miles and cleared to land, he forgot to extend the landing gear. These things happen. With landing flap down and horn silenced we were faster than normal (no gear). As the instructor I had to either say nothing or tell him to ask for gear down while there was time - or go-around. I was counting on the flashing ASI flag as a last ditch reminder of the gear being unsafe and hoped he would take the required corrective action after he saw the flag.
But to my amazement he was concentrating so much that he never saw the flashing flag. Or, if he did, it meant nothing to him because he was new to type. He pressed on trying to get the speed back towards VREF.
At 500 feet I told him to go-around on two engines as I zeroed the rudder trim. He did as he was told and called for gear up. I told him to raise the gear himself instead of me doing it. He reached over to the gear handle and saw it was already up. That shook him to the core. It was a lesson well learned for both him and me.
I had seen with my own eyes how even with the most reliable of multiple gear warning systems, (horn and gear lights and ASI flag) can be missed when the pilot is concentrating on something else. In this case approach speed and profile.