I do not know the exact mechanism how those files are uploaded but c:Category:Pages_with_script_errors often has files without license but with lua errors newly uploaded by UploadWizard from wikipedias presumable using some campaign interface.

Today's example files:

• File:Javier_Rapper_In_Tehran_02.jpg
• File:RASK Jewelry no CO2 IKON GREEN on white.png
• File:جلد راه روسیه 1.jpg

Those uploads usually have very similar wikitext, including:

• {{self|subst:Custom license marker added by UW}} - this statement causes lua errors
• {{Uploaded from Danish Wikipedia}}, {{Uploaded from Persian Wikipedia}}, etc.
• [[Category:Uploaded via Campaign:dk]], [[Category:Uploaded via Campaign:fa]], etc.

At the minimum do not upload files with wikitext which causes lua errors, and ideally do not upload files without a valid license template.