I remember working in a dev team where only one person knew where all the keys for all services were stored
If he was hit by a bus, we were f*ed
I had a team where we always talked about our bus factor - how many of us it would take to be hit by a bus to royally screw the team. We also got nervous when too many of us shared an elevator together, hah.
In all seriousness though, knowledge silo'ing can be a nasty problem