Given a limited resource and a limited time to test a product or service, it will never be possible to test every permutation and combination of every element of functionality.
You focus test development and execution on those aspects of the product or service that are more likely to fail and impact the users, rather than testing those that are sure to work or those that if they failed would have no impact on users.
The votes have been tallied, the results are in, and we have a winner (two, actually). Congratulations to the newly elected user representatives to the LJ Advisory Board: from lj_election_en, it's kylecassidy, and from lj_election_ru, it's nekbke! You can see the full breakdown of results for lj_election_en here and for lj_election_ru here. Thanks to everyone for your participation.