We've been having this error pop up in our Marketo instance after completing the Marketo-AdWords integration.
Here's Marketo's comment on the matter:
OfflineConversionError.UNAUTHORIZED_USER - this error means this gclid (the ad it posted) did not come from that account. It won't allow you to upload conversions for a different customer that you don't manage.
Google's comment is not worth posting.
Has anyone had a similar occurrence? Is it that we have had an offline conversion for a person who has interacted with an ad of ours, but from an AdWords account not linked to the one that is linked to our Marketo integration? Any insight would greatly be appreciated.