In reply to Duncan Disorderly:
The day I did Black Mamba, me and my partner were the first folk at the crag and had peace and quiet for the first couple of pitches. The next party to arrive, for reasons best known only to themselves, decide that they have to do the same route as us (bearing in mind that the Creag an Dubh-Loch is over half a mile wide and 1000 feet high).
Route finding on Black Mamba is not straightforward, particularly on the pitches below the terrace, so with us showing them the way, this second party catch us up. I'm belaying at this point, with my leader sussing out where the next pitch goes. The other leader asks if he can climb through and take a higher belay, and trying to be friendly, I tell him to go ahead. From the higher belay, the other party's last pitch to the terrace is shorter than ours and they get there first, and so are ahead of us as the route continues on the upper slabs.
On the second pitch above the terrace, they go the wrong way and spend over an hour sorting themselves out. I had set off up below them and ended up having to clip into my gear and sit on the rope as there was no more room for me at their belay. During this wait, the weather started to turn, and seeing no immediate prospect of them getting back on route quickly, I ended up climbing back to the terrace and we ended up traversing off onto Central Buttress, meaning that we never actually finished the route.
I was partly annoyed at the other climbers for stopping us finishing our route, but I was also annoyed at myself for letting them climb through in the first place. From a safety point of view, I shouldn't have let the leader take that higher belay above me while my leader was climbing, since anything knocked down from above onto me by that other party woudl have compromised both my safey and that of my leader, and there's no justification to let anyone do that.
In future, I won't let anyone climb through in that sort of situation again.