Steps to reproduce:
- Have a content model with multiple formats
- Edit the page, passing a non-default format with the url parameter &format=foo
- Hit show changes
- Expected behaviour: You get a diff
- Actual behavior: MW seems to try to serialize the contents of the edit box as if they were the default format. This fails. Any subsequent preview/save button presses also fail.
Note that most normal editing/preview etc work fine with non-default format.