Over the years and in no particular order:
taking off with carb heat
leaving fuel caps off after refuelling...and flying after...
doing up the harness sometime after take off
leaving the pitot cover on
leaving the rudder lock on
not checking the primer locked
spare seat belt flapping outside
taking off with full back trim
getting strapped in and started before realising the headset is in the car
and probably several/many more but I'm getting older and can't remember...
None of the above caused more than a
moment, except the fuel caps, which was a grovelling job as it wasn't my aeroplane. The confessional revealed that the multi-thousand hours career instructors have as many screw-ups and just because we've admitted a few, it doesn't mean we won't make all the rest and then some.
So despite the checklists and good intentions, we are but the stuff of Human Factors.
Happy flying year to all.