I would honestly admit that I probably make at least one mistake on most flights - something as simple as missing an item off a checklist, keeping head down too long in the cockpit, not checking regualrly enough from map to scenery and vice-versa. Mostly small and inconsequential.
Problems usually occur when you make several mistakes that build on each other - classic for me was being rushed in my "A" check and leaving the pitot cover on, this was exacerbated by the cross wind being just below my limits and the third was not checking that the indicated airspeed was live during the take-off.
Result - in the air with no airspeed indicated, cross wind limit now increased beyond my experience and capabilities. No terrible results as I flew on to my destinatin where the wind was straight down the runway and landed using power + attitude to set airspeed for the approach.
Sweaty moment though which COULD have had a much worse outcome if (for example) I had another problem to add to my mistakes.
The trick is to LEARN from mistakes, it won't stop you making them and making new ones, but hopefully it will reduce their frequency and avoid mutiple issues on one flight.
My two pence worth.