Generally, the XQuery update specification is written so that dynamic errors are detected as early as possible: either when the updating expression itself is evaluated, or when testing the consistency of the updates within the Pending Update List. There are two exceptions, errors that are described as occurring during the application of update primitives.

The first is in upd:delete: "If $target has no parent, the XDM instance is unchanged. An implementation may (but is not required to) raise a dynamic error if $target had no parent before execution of the query began. [err:XUDY0020]." This error condition belong much more naturally in the rules for the delete expression (and indeed it is mentioned there, though only in a Note).

The second is XUDY0024. This occurs when two update primitives in the PUL would cause conflicting namespace bindings to be added to the same element. (It is phrased as if it also occurs when a new namespace binding conflicts with a namespace binding that was there before the update started, but in fact all such cases are caught earlier, as XUDY0023.) XUDY0024 can be expressed as a consistency constraint on the content of the pending update list. All other such constraints are described in the rules for upd:mergeUpdates and upd:applyUpdates. 

Does this make any operational difference? In the absence of try/catch, perhaps not. But with try/catch it becomes significant which expression has failed, and an error described under upd:mergeUpdates() can be raised during the evaluation of the expression that caused a primitive to be added to the PUL. Also, of course, some implementations may find it helpful to perform all the consistency checks on the PUL before applying any updates, as this makes it easier to achieve atomicity; it would therefore be helpful to readers to describe XUDY0024 as a constraint on the consistency of the PUL.

The error could be described like this (in 3.2.1 and 3.2.2):

e. Multiple namespace bindings are created for the same element by primitives on (the merged list | $pul), such that two or more such namespace bindings have the same prefix (or absence of a prefix) and different namespace URIs. Primitives that create a namespace binding are:

  i. upd:insertAttributes creates one namespace binding on the $target element corresponding to the *implied namespace binding* of each attribute node in $content

  ii. upd:replaceNode creates one namespace binding on the $target element corresponding to the *implied namespace binding* of each attribute node in $replacement

  iii. upd:rename creates a namespace binding on $target, or on the parent of $target if $target is an attribute node, corresponding to the *implied namespace binding* of $newName.