When test case status is changed to passed then it is not possible to revert it back to untested status - so no way to correct mistake.
That’s actually be design, so that you cannot reset/change a test after having added some results (this can be important in larger teams and/or for auditing purposes). This is very similar to how many bug tracking tools do not allow you do delete cases.
That said, you can always set the status to ‘retest’ in case you have accidentally changed it (which means effectively the same thing as untested, i.e. the test needs still to be tested).
Maybe it would be a good idea to add an option to revert/delete test results within, say, a certain period of time (similar to the edit option for test results). I added a feature request for this.
I think this is minor issue at all. But I would rather expect admin profile to be capable to do this.