When Should You Retire A Carabiner?
A lot of climbing protection systems have some built in redundancy, but it is important to keep in mind which parts of your system have limited redundancy and what condition those parts of the system are in. Aluminium carabiners don't last forever. The wear from them 'notching' on bolt hangers or 'grooving' from rope wear is easy to inspect visually and worn 'biners can be completely retired or moved into non-critically load-bearing use.
If you spend enough time at a crag you'll see someone drop a quickdraw while racking up their harness and someone nearby will chime in that the dropped carabiners should now be retired (usually followed by an offer to take possession of the retired gear). The possibility of microfractures in carabiners lurks around climbing circles like a folk legend, with those new to the sport wondering if it is a trick by old-timers to scam them out of their gear.
The UIAA Safety Commission regularly investigates safety matters related specifically to climbing and makes their findings publicly available in a series of articles. Their latest post addresses the problem of very hard to detect fractures in aluminium carabiners resulting from high speed impact. While their findings don't necessarily make it any easier to know when a dropped carabiner needs to be retired, it is important information to know and good background knowledge for when you think someone is trying to scam you at the crag.