Common mistakes in usability testing include:

Ignoring critical results (ii): A significant issue if findings are not acted upon.

Scheduling the test too late (iv): This can lead to usability problems being discovered too late to be addressed effectively.

Having the wrong focus (v): For instance, focusing on aesthetics instead of usability goals.

Option i ("user is too active") is not a problem—active participation is necessary. Option iii ("no observers involved") is not a typical mistake, since tests can still be valid even with recorded sessions and later review. Hence, the correct answer is A: ii, iv & v are true.