Somewhere between move 10 and 20? Maybe a bit earlier, if you play super-sharp stuff like Sicilian mainlines, or something provocative like the Pirc, something off-beat like Owen's? Well, I can out-do you, all of you. Because I've blundered in this position before:
That's right, I've blundered at move zero: I've blundered before a piece or pawn has even been touched: I've blundered right there in the initial position. And not only that, I've done it twice.
The first time was at a Surrey League match toward the end of last year, and my opponent was late. I had black, and I sat merrily watching the clock on my left tick his time away - 5, 10, 15, 20 minutes - and finally, he arrived. We shook hands, he sat down and played 1.e4, at which point I noticed the clock was on the wrong side. So I switched it over to my right, played 1...c5, and pressed it. All jolly good, except a few moves later, in a super-sharp mainline, I looked to my right and wondered how I could already be half an hour behind on the clock.
The second time, a London League match earlier this year. I was on board 6, and when I arrived a bag was on the seat of the white player - my opponent's bag, I assumed. So I put down my pen on the black side, started his clock, and strolled around waiting for him to shift his bag, sit down, and make a move. Except he didn't, because when he arrived quarter of an hour later, he pointed out that he had the black pieces, and that I had the white pieces, and that no, that wasn't his bag anyway. In retrospect this should have been rather obvious, since it was sat between two of my teammates. But alas as usual, I was all Watson and no Holmes; every bit a Hastings and not a jot a Poirot. Rather nobly, he offered to rewind the clocks to reflect his lateness, but of course that's just not fair. It was my turn to move, and thus I had taken fifteen minutes to decide upon 1.e4.
The moral of the story? I don't know. Maybe I could work it out, if only I had given myself more time.