As I said, I do not think that slime is in any direct peril but those experiences made it hard for me to trust darcs, all its other nice features aside.
this is due to an exponential algorithm in darcs, but it only happens when darcs tries to merge some badly nested conflicts. it only happens in "normal" usage when you have branches that get too far from each other by touching the same files at the same positions too much.
this is a structural weakness of darcs, but the good features come from this internal operation, too.
the guys at #darcs said last time i ran into this problem that they are planning to bail out from such an exponential situation and mark the conflict for user resolution (which would happen anyway after 10^n years... :)
haven't seen this change yet, though.