Steps to reproduce:
- Open an image-containing page in VisualEditor on a non-English Wikipedia (e.g., Afrikaans).
- Open a page on any Wikipedia that doesn't use that language (e.g., English).
- Copy the image from the non-English Wikipedia.
- Paste it into the English Wikipedia page.
- Everything looks fine here. Save the page.
- See the red mess where your image was supposed to be.
- Open the wikitext editor and fix the File: namespace.