I rooted some Christmas cactus cuttings from a healthy plant over a year ago. Recently, they've been dying.
I did find a possible culprit: fusarium root rot/wilt. Not sure if it's salvageable though.
Some info on Fusarium oxysporum in Christmas cacti. It's a fairly recent scientific article, but I can't find anything else on this topic: http://www.apsnet.org/publicat...
The only flaw I see with fusarium is that it tends to kill a plant very quickly, as far as I've heard. My cacti have been showing problems since I rooted them in soil last year. The 'bite' on the leaf was the first spot to appear when I rooted them. I uprooted the dying one to examine the roots and there are no signs of rot, so how the rot may have ended up in my cactus is a bit of a mystery. The cuttings were originally rooted in water, so it's possible that they were weakened by that.