I've had this exact situation before. For me it's usually with written reports. I've had certain customers who when I send something to them for review and sign off they will always find something. When I'm dealing with someone like that its easiest to leave some minor non-techical detail on the first page for them to find, otherwise they'll randomly find something to argue.

A few examples of things customers have come back on documents with:

-grammer changes, not corrections but just rewording sentences.
- changing content that is referenced From other signed documents... " No you can't change what that says... It's a reference from a document you signed last week." -removing document references, ie. Call-out to work instructions that list the template, call-outs to internal docs like quality manual, etc -Trying to remove part numbers and revisions.
-I've actually had a customers go through a technical report and using the MS Word markup tool, change the spacing between sentences from two spaces to one space.