[thwg]

A few years ago there was a case in China. Someone forged documents in the name of a mayor, using a seal with the mayor's name on it. The authorities later found it fake because the mayor used a different seal design. (Just a random detail in a complicated lawsuit.)

It would have been difficult if nobody else had seen his real seal though.

[bruxis]

For important hanko seals used in Japan (for business, loans, etc.) they are registered with the local ward office so that wouldn't be an issue.