I have a Procurve 2626 on a small network (~200 users); about 100 of them are one port, which every now and then sets itself to "off" in the running configuration.
There is no indication of why this happens in the logs.
From searching the net and HP support forums, it seems others have this problem too, but no solution seems to have been found.
As it is extremely difficult to rewire the network, using multiple links for the users is not possible. (The oldest parts of the buildings are from the ~1625, so just thinking about rewiring is virtually a no-go...)