How to reproduce:
- Go to any revision history.
- Click the datestamp on an old version.
- Click the edit tab.
- Scroll down and click the cancel button.
What happens: The current version of the page is displayed.
What should happen: The old version of the page (same as step 2) is displayed.
Workaround: use the back button in the browser, instead of the cancel button.
(Seems so obvious that someone should have posted it already. But I searched several times and didn't see it.)