Some time ago I was teaching a studio portrait workshop. I set up some lighting and talked about engaging the subject. I rotated the students through being the photographer and being the subject to get perspective on each.
Toward the end of the session, I started to talk about alternative lighting setups and used one of the students as a model. I snapped a quick shot to fire the strobes as much as to take a photo. When I showed the image on the rear screen of the camera to the student, he loved it and wanted a print. Sure not a problem, until I got home and realized I had missed the focus by a bit. The lighting was also not quite what it should have been.
I did fix it a bit in Photoshop, salvaging something the student would be happy with, but I learned a lesson. ALWAYS take the best image possible. Treat every shot as if it is important. You never know what you may end up with.