This is a possible multi-threading problem that could affect XQuery or XSLT.
The method Expression.getSlotsUsed() is called at run-time to determine the dependencies of an expression on local variables. The method computes and saves its value the first time it is used. It should therefore be synchronized. The possible effect of not being synchronized is that one thread detects that slotsUsed is not null and therefore assumes that it is safe to use its value, when in fact the thread that is computing its value might not yet have finished doing so. This would mean that when creating a Closure during lazy evaluation of a variable, the values of all dependee variables are not saved in the Closure, causing an AssertionError (value of variable is undefined) when the value of the dependee variable is subsequently accessed.
A patch is being raised in Subversion that causes this method to be synchronized.