In XQuery, when a computed attribute constructor is used and the attribute name is supplied as a value of type xs:QName, there is an error in the code which manifests itself when multithreading. The effect is that the attribute may be written with the wrong URI. The error could also cause two attributes on an element to be given the same name, resulting in a dynamic error.
A patch to module net.sf.saxon.instruct.ComputedAttribute is being placed in Subversion. (The error is that the computation of a namespace URI for the attribute uses the instance-level variable "namespace", which is shared by all threads; it is therefore possible for one thread to overwrite the value set by another thread before it is used.)
The bug also affects 8.7 and 8.8, and probably previous releases.