This video, made in the 90's, explains how bolt hangers can damage karabiners. The sharp edges left on karabiners can cause serious damage to ropes. For this reason, never use your bolt clipping 'biners to clip into the rope.
Extenders or quickdraws used for sports climbing should always have the same end clipped into the bolt. Some bolt hangers can damage the karabiner, leaving rough burrs or edges. A rope clipped into such a karabiner can be seriously damaged, and possibly even severed in the event of a fall.
The solution is to use distinct karabiners for clipping into the bolt and rope, for example:
Straight-gates for the bolt, bent-gates for the rope
Have the rope clipping 'biner kept captive by a rubber band or "String"
Use colour codes, such as Blue 'biners for Bolts, Red for the Rope
In addition, make sure you rack your extenders on your harness the right way up so that you don't have to think about it in extremis!
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