As an Operations consultant, I always see people focus on redundancy on all levels... Except for documentation.
For some reason everyone says, there should be documentation, but never ends up being usable documentation. Either it's too much, too little or something else :)
I believe one should view documentation as redundancy, on the people level. If the guy who usually does X, is not there - someone else needs to be able to do this. That means some level of documentation needs to exist (or you need to ensure that others know the exact same as you - which really will never work - and it's highly error prone).
If you view documentation as a means of getting people redundancy - then you wouldn't setup anything redundant, without actually testing that the redundancy works, from time to time.. - would you?
To test documentation, you make sure that no one task, is only performed by one person - but others get the task as well - and WHEN the others find the documentation lacking, they are allowed to ask the guy who knows the answers - AND they MUST update the documentation immediately. If they ask again (or someone else has to ask, because documentation was not updated) - the one being asked, is entitled to cake, coke or whatever :)