Seen at enwiki:
- Find an edit you don't like.
- Click the undo button. You'll (probably) end up in the 2010 WikiEditor. Do NOT make any changes in the wikitext mode.
- Switch to the visual editor.
- Make another change. (If you don't, then the 'Publish changes' button will be grayed out.)
- Save changes. Note, with satisfaction, that the complicated default edit summary has been preserved.
- Discover that the reversion didn't happen. Everything that you wanted to revert is still on the page.
If you've gone through the above steps, an additional problem is that the next time you click on an undo link on the revision history page, you will default to VisualEditor. If you then switch back to Source editing, you're redirected to wiki/Main_Page?action=edit&veswitched=1&oldid=24 instead of w/index.php?title=Main_Page&action=edit&undoafter=23&undo=24 (the undo and undoafter params are lost), so you're editing the page rather than undoing a specific edit.