- Set an edit filter to disallow edits on a particular talk page
- Switch to mobile view, and attempt to reply to a comment on that page (Do NOT use the "read as wiki page" option)
The expected result is that the message you selected for the filter is displayed.
The actual result is just a blink-and-you'll-miss-it "Error, edit not saved." and nothing else.
When adding a new section, it's "The topic can't be added due to an unknown error." instead.