This bug only occurs when logged-in as an administrator. When viewing a history page, selecting 2 versions and clicking Enter brings up a "Invalid version" page.
This happens because the "Show/Hide marked versions" is the default submit button for that form. Instead, the "Compare selected versions" should be the default, like for non-sysops.
Going further, it would be a nice enhancement if the "Show/Hide marked versions" button would become default when a version is actually marked (the checkbox is checked). If not, the default should be "Compare selected versions".
Version: unspecified
Severity: normal