Finally understanding what the 'freezing level' in the forecasts actually means.
then rapidly descending to what little warm air was still above the ground and watching ice come off the prop and hit the windscreen.
Always remembering to apply magnetic variation when flight planning.
otherwise one can fly fat, dumb and happy for 2 hours, 10' off track, before realising that the map reading up till then has been an entertaining work of fiction.
The planned TAS and wind-corrections will never match reality.
therefore ensure that you have more than sufficient fuel to get to somewhere with fuel