In 2004, I was flying from Wangen Lachen (LSPV, Switzerland) to Corfu (LGKR Greece), VFR.
I had to climb out OCAS to FL129, in a small triangle of airspace, in an orbit all the way up, to cross the Alps. (Zurich would not allow me into Class C, base FL130).
I got to about FL100 and the thing would not climb any more. No icing, flaps away... Had a look around... the gear was still down
It was the result of a stressful departure situation, where I could not find anybody to pay the fees to, could not find anybody to operate the avgas pump, had the wrong currency on me, could not get the laptop fax to work to send the PNR Customs notification (LSPV has "Exit Customs" only) so had to use some fallback method for that (early days of electronics comms back then).
Once or twice in 10 years I left the key in the mags, in the ON position, during refuelling, which is really stupid.
As Fuji says, not many things (in fixed wing ops) are actually critical. A high perf airplane also helps.