Symptoms

"Process data field 'datafieldname' does not exist" error when deploying workflow. The user had a bunch of string data fields being used within workflow task events, and later removed the data field references and the data fields themselves.
 

Diagnoses

Saving out a copy of the WKPRX file and opening in a text editor to examine the XML showed that the data bindings appeared in what looked like some structure for a user task's Data Bindings section.

After examining the Data Bindings section in a User Task, the referenced data fields were found buried in all the other fields. They had been added via one of the local views in the output mappings section for workflow task rules, which at some point must have been absorbed into the workflow's XML, and were not removed upon deleting the data fields.
 

Resolution

Remove the invalid data fields from the output mappings of workflow related view / form rules and redeploy.