Tor will use an unconfigured bridge if it was a configured bridge last time you ran Tor
If you configure your Tor client with
usebridges 1 bridge 184.108.40.206:9009
and you run it and it works, then Tor will end up writing two things to disk: 1) a @purpose bridge descriptor for 220.127.116.11 in your cached-descriptors file:
@downloaded-at 2011-02-08 07:54:52 @source "18.104.22.168" @purpose bridge router bridge 22.214.171.124 9009 0 0 ...
and 2) an entry guard stanza in your state file:
EntryGuard bridge 4C17FB532E20B2A8AC199441ECD2B0177B39E4B1 EntryGuardAddedBy 4C17FB532E20B2A8AC199441ECD2B0177B39E4B1 0.2.3.0-alpha-dev 2011-02-01 18:43:23
Then if you kill your Tor and run it with
usebridges 1 bridge 126.96.36.199:9009
it will successfully bootstrap -- using the bridge that worked before but isn't your requested bridge.