Q. Is it okay to use carabiners that have been dropped?
A. Unfortunately, the only way to know if “dropped” carabiners are fit for use is to test them to their breaking point. This doesn’t do you much good, now does it? It's best to inspect dropped gear for dings and significant trauma. If only light scratching is visible and gate action is still good, there is a good chance it is fit for usage. Remember, only you know what your gear has been through and if there is any doubt, it's best to retire the gear rather than take a risk.
This myth is certainly something you hear climbers talking about all the time, and there's a constant paranoia about gear that people have dropped onto rock or concrete floors at gyms.
When did this myth start? Has there ever been a case where a piece of protection has failed because of micro-fractures?