...never throw out or sell a biner unless it just won't function at all. That is just my thoughts.
Training is the key Burnham. That is something that is sorely lacking in the private sector.
But I can tell you that I did receive some training by a very reputable climber (who shall remain anonymous) and during the demonstration of fall protection caught his own mistake of a screw gate carabiner being all the way loose instead of tightened.
If a seasoned climber can screw it up (no pun intended >>> no, really!), then I surely won't trust that piece of gear. Ever.
No record of an accident caused by this = it's never happened ???