Turned the fuel selector in a DR400-160 to "off" when it should have been going to "center". Luckily the passenger (a pilot too) sitting next to me caught it and corrected immediately. We were at 1000 feet at that time, and that would probably not have been sufficient to recover.
In my defense, in the DR400-160 (with the saddle tanks) the fuel selector points *backwards* when selecting "center" and *forwards* when selecting "off". AFAIC, a gross violation of the "forward or upwards is good/go" principle.
Forgot to turn the mags off when doing a quick passenger change. No excuse there.
Forgot to turn the transponder on when entering the runway - multiple times. Forgot to turn off the carb heat after landing - multiple times. Same for landing lights & transponder. Have now altered my routine for entering and leaving the runway.
Changing my navigation plan laterally in-flight (due to intense glider activity at a certain field) but kept executing the plan vertically, thus busting class C. Just a corner, no harm done except to my ego.