Out at the crag the other day, my partner (Brett) and I had a near miss that I am still unable to explain.  Any theories on how this incident occurred would be welcome.

We were using what I regard as pretty standard toprope setup.  Static rope fixed to an anchor point, follow rope from tied end to a point where it will hang clear of edges/rocks etc. Tie two figure eights on a bight close together creating two relatively small loops side by side.  Check that they will hang free of cliff edge.  Take other end of rope back and tie to a second anchor point.  Two opposed locking biners are each passed through both of the side-by-side loops.  The middle point of the climbing rope is then passed through both the opposed, locking biners (but not the static rope loops).

The set up was checked at the top after the climbing rope was tossed down - no twists and everything clear (looking down, I could see both lengths of static rope, then the two figure eight loops, then the biners, then the climbing rope).  We descended via nearby steps.  At the base of the climb, I checked for twists in the climbing rope (looked up to the biners and could see the rope running cleanly through both).  I then pulled some of the climbing rope through to take up slack from the end at which Brett would tie in - again, the climbing rope was running cleanly through the biners.

Brett and I then took it in turns to climb the route immediately below the biners and were each lowered off after reaching the biners.  Neither of us noticed anything peculiar about the biners or ropes (we usually climb up to and touch the biners).

Using the same set up, I then climbed a route slightly to the left of the first route and was lowered off.  Again, I climbed almost level with the biners.  I must have looked at them (don't recall), but didn't notice anything.

My partner then climbed the route and at the top he called out to me that it looked like there was a burn mark on one of the loops in the static rope.  The static rope we use is predominantly white.  I dismissed this and called out that it must be the grayish stain which is caused by the aluminum biners and which is frequent source of black palms for the person belaying.  My partner was insistent but said he felt it would be OK to lower off it.  I lowered him and we then went to take a look at the anchor setup.  Sure enough there was a burn mark on one of the loops.  The burn was serious enough to have melted an area of the outer casing of the rope (although it was not melted through to the core).   If you imagine the loop as a clock (with the biners hanging at the 6, the area of the burn was at about the 10).

We were lucky, because (we think) we only had one lower where the ropes were running against each other.

We were astonished, because we could not work out how the climbing rope could come to be in contact with the static rope loop at the 10 o' clock point in a way which clearly applied all or most of the load to that contact point.  We were also shocked by how quickly you can seriously damage your rope.  Bear in mind that this damage was caused by the friction generated during part or all of a relatively slow lower (we are not kamikaze descenders).

Lessons of the day <subject to the usual gumby disclaimers>

Only a dope allows rope to run over rope.

Check that the rope is running cleanly through the biners before starting and before lowering off every climb.

If you have to cross over/under the other end of the rope or make any unusual sideways moves, check whether this has any impact on how cleanly the rope is running through the biners.

Listen to your partner.

Expect the unexpected <another useless platitude from the gumby>