MarkWLItemWorked - The current worklist instance was not marked 'worked.' (2,151)

A small percentage (roughly 1 per 2500) of executions of peoplecode function MarkWLItemWorked() are failing to mark the worklist item as worked and produce this message in the app server log -- "The current worklist instance was not marked 'worked.' (2,151)".

The MarkWLitemWorked function is being called from my custom peoplecode.  I cannot reproduce the error.  It occurs to different users at different points in the approval process.

Anyone have experience/knowledge with this delivered function and/or error message that might help me determine the root cause?

PeopleBooks states that MarkWLitemWorked returns a boolean but does not explain what situations would cause it to return a False.  GSS support is looking at this but are not optimistic and since the call to MarkWLitemWorked is in a custom peoplecode program they are limited in their ability to help.