As requested.
While conducting training on a new hire just prior to letting him loose on the customers I elected to give him one last surprise engine failure just at the limit of our range to reach the runway, so a max-range full-down in an R22. Unfortunately, at dusk.
As he drooped the rotor to '90' to extend the range everything looked good, then started sinking low. I looked across and he had all parameters perfect, so I let him continue. It was getting darkish, late dusk, so it was a little tough to see precisely the gauges on his side of the cockpit.
We were no longer going to make the runway so I took control to overshoot and figure out what he did wrong. I was still quite comfortable until I rolled on throttle and the engine only growled with insignificant acceleration. A little panicked I leaned over to look at the RRPM and saw he had been at 80 not 90! A sharp flare allowed the RRpm to increase sufficiently to get the engine back and I recovered in ground effect.
No more late in the day training flights where eyesight gets weak and fatigue dulls the senses. No more casual glances to confirm the students actions.