It happened to me.
Many years ago, as a brand new CFI, I took off without a clearance after being given the position. The student had about 15 hours in airplanes and we were beginning to learn short and soft field t/o and landings. We started with the short field takeoff, which was previously briefed. We were holding short of 25 at KORL and the controlled cleared us onto the runway for position and hold. As we took the runway I talked the student through the line-up and power up. Just as we had briefed, I instructed the student to release the brakes and continue down the runway.
Just about eight seconds later, the tower lady came on screaming at me! "N12345, were you cleared for takeoff?!?!" In fear for my certificates, I panicked and instructed the student to abort the takeoff. The student, also being new, slammed on the brakes resulting in a blowout on the left main. So, there we were on the side of the runway with the runway shut down for thirty minutes.
It is kind of a funny story between the two of us, even to this day. More importantly, a lesson learned from a mistake that anyone can make.
Although, this begs the question(s):
How many mistakes is too many for a professional pilot flight crew operating in dense traffic environment? Is just one mistake too many?