And that is why I account for every single one of my tools as I work a specific part of the job. Leaving tools scattered all over the place leads to that kind of a mistake. It's sloppy and unprofessional IMHO. These are mistakes that could be potentially dangerous, especially on two wheels. Don't get me wrong; I'm not perfect by any means and have made more than my share of errors over the years. However, my old-fashion, old-school slow method of working leaves no wrenches, sockets, tools, etc. unaccounted for. Heck, I've even found tools wedged up in bikes from previous service visits to other shops!
Before my customer's bike is wheeled out of my shop, ALL my tools are accounted for, cleaned and put back in their proper trays. Everything is double-checked and I go over my findings on the bike with my customer before he leaves. I let him know what I found and the seriousness of it. It's his choice when/whether to have it repaired of live with it, but my job is done and my conscience is clear.