http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/236-single-guard-node.html
This behavior _might_ make some attacks harder; for example, an
attacker who shoots down your guard in the hope that you will pick
his guard next, is now forced to have evil guards in the network at
the time you first picked your guards.
However, this behavior might also influence performance, since a
guard that was fast enough 7 months ago, might not be this fast
today.