I've got two first solos in my logbook.
First one was in N230FT, a PA-28-161. I taxied out (with the intent to fly, obviously) but the aircraft failed the run-up checks - mag drop of 200 rpm on left. Tried to clear it, to no avail, so had to taxi back. But since I moved the aircraft with the intent to fly, I could write it in my logbook.
The second was in N250FT, about 15 minutes later. Three full-stop-taxibacks without any problems. I don't remember much of it anymore. I guess I spent all my adrenaline on the first first solo. By the time I taxied back, N230FT already had its failed spark plug replaced.