Mobile talk page editor-thing doesn't show CAPTCHAs (on Wikimedia wikis, new users who add an external link in their edit must fill out a CAPTCHA). Instead it acts as if posting the reply was successful and refreshes the page, even though the reply was not saved. The user loses anything they typed in.

I learned of the problem this morning, when an annoyed user came on IRC complaining that someone was "deleting" their talk page posts (they were linking to a page that was a source/reference for their reverted edit). After I guessed what the problem is, they were able to switch to the desktop interface and eventually reply that way.