Assume one page Test has a history like:

• 20160101000002 UserB content=2 (rollback)
• 20160101000001 UserB content=1
• 20160101000000 UserA content=2

When the rollback link is clicked, it tries to roll back to 20160101000000 where there's no content change, thus a failure is thrown. However this error is represented as:

Rollback failed
Cannot rollback last edit of Test by UserB (talk | contribs); someone else has edited or rolled back the page already.
The last edit to the page was by UserB (talk | contribs).

This message is confusing because there isn't anyone editing the page in meantime.