If regeneration fails, there is posted error of severity DocumentCorruption that will be delivered to the end user at the end of transaction explaining what specifically has happened.
If regeneration has failed, document is corrupted and even reading from it is illegal.
Code that called regeneration and got this exception should not catch and ignore it - instead, it should do nothing but internal cleanup and return control to the owner of currently active transaction/subtransaction, which must be aborted.
Modification of the document is forbidden. Typically, this is because there is no open transaction; consult documentation for Document.IsModified for other possible causes.
Remarks
Use this method to force update to the document after a group of changes. Note that when a transaction is committed there is an automatic call to regenerate the document.