Mine was at Perth (EGPT), where I used to work. As you can see from the link, the runways there are laid out in a triangle, and on this day, like most days, we were using 21 - so we were holding on the grass up by the threshold. The run-up was all done, and we were ready for departure.
Then Air Traffic changed the runway to 27 (which was then 28). This would've involved our taxiing back to the threshold of 16, then down the "grass taxiway" which used to run to about the midpoint of 27, holding to cross 27, taxiing all around the "kidney" taxiway and finally bouncing over the rough grass to holding point D - and that's what Air Traffic told us to do.
My instructor turned us round and started us on this trek. As we passed 16, Air Traffic called us up: "c/s, you can use the grass runway if you like." My instructor replied, "c/s thanks," and lined us up... then just kept chugging along, not adding any power, and - no flaps, for a soft field take-off? Huh? What's he doing? Oh.
Had I been solo, I'd have taken off without clearance, right across the landing traffic for 27.
Of course, it's never as simple as "shouldn't have said that", "shouldn't have done that"... These were the things I thought were pertinent:
- "...use the grass runway..." - what else do you use a runway for?
- As I say, I used to work there - so cups of tea in the tower and first-name terms all round. That, and the fact that two qualified pilots didn't need the same mollycoddling as the others on the frequency (early solo students), probably contributed to the informality.
- Not only was I newly qualified, but this was only my second or third flight out of Perth... and it took many more before my instructor and I agreed I should be turned loose, due to the difference in procedures and sheer volume of traffic.
- In fact, I'd qualified abroad and had rarely encountered ATC at all. In 55 hours, I was "cleared for take-off" 3 times - those magic words just weren't something that even occurred to me as being necessary yet.
So quite a few contributory factors, but the non-standard R/T would've set the ball rolling. I'm just glad my instructor was driving.
I was very quiet for the rest of that taxi - and for most of the flight.