The Bus Factor for a project is the number of people that if hit by a bus (or equivalent leave) would break a project.
In an ideal situation there are sufficient runbooks and documentation for a project to continue should all the team members leave. This is frequently not the case.
Pairing can help reduce the Bus Factor. Two people have context on most things. This can still go wrong with the minimum effective team size (3). METS allows for one to be on holiday, one to be sick and still have someone to continue some form of progress. The downside is that someone will not be pairing (or the team can only mob on a single item).
You can’t eliminate the Bus Factor but need to reduce it. If anyone has too much information in their heads then they need to start actively writing down more than they would normally do. This means carefully documenting how to do anything that only they need to know.
This can seem counterproductive if having special knowledge makes you feel special. However being unreplaceable also means being unpromotable as the essential knowledge that you hoard will become a burden.
How big is your projects Bus Factor?