Most developers will be familiar with the Bus Factor.
The "bus factor" is the minimum number of team members that have to suddenly disappear from a project before the project stalls due to lack of knowledgeable or competent personnel.
The problem is it isn't always a simple thing to overcome in a development team whether it be someone on holiday, off sick or any number of things that mean someone isn't around to share their knowledge.
What is your approach to increasing your Bus Factor? How do you make sure that the whole team is brought up to speed on projects without reducing capacity too much?