This type of incident is more likely to happen with the first series of ascenders having a nylon trigger, introduced in 1998. Specifically, those with the following serial numbers:
B17 R from 97206 to 99109,
B17 L from 97253 to 99091,
B18 from 97308 to 99112.
The safety catch was redesigned in 1999, so recent models do not have this problem nor do the earlier models with metal safety catches.
The Petzl website reports the same incident thus:
NB Petzl seem to down play the problem by talking about slipage, whereas Descent reports that the ascender actually came off the rope.http://en.petzl.com/petzl/SportNews?News=142 wrote:Improper loading was the likely cause of a recent incident in which a caver was ascending a flowstone slope. He was using an ASCENSION (attached to a lanyard) for self-belay/aid on a fixed rope, walking up the slope with his feet while using the ascender as a hand hold to pull himself up. At some point during the climb, the ascender slipped down the rope and the caver fell 7 meters (without serious consequences).
For further information see the above link and Descent 190.