If you start Tor with UseBridges 1, no ClientTransportPlugin, and only obfs4 bridges, it should fail
If you start your Tor client with just this line:
it will refuse to start, because you gave it no bridges:
Jun 13 00:06:48.337 [warn] Failed to parse/validate config: If you set UseBridges, you must specify at least one bridge. Jun 13 00:06:48.337 [err] Reading config failed--see warnings above.
But if instead you start it with
usebridges 1 bridge obfs4 22.214.171.124:443 FC259A04A328A07FED1413E9FC6526530D9FD87A cert=RutxZlu8BtyP+y0NX7bAVD41+J/qXNhHUrKjFkRSdiBAhIHIQLhKQ2HxESAKZprn/lR3KA iat-mode=0
it will start up, trying to treat that bridge as a vanilla bridge:
Jun 13 00:10:23.164 [notice] Bootstrapped 0% (starting): Starting Jun 13 00:10:23.278 [notice] Starting with guard context "bridges" Jun 13 00:10:23.279 [notice] Delaying directory fetches: No running bridges Jun 13 00:10:24.153 [notice] Bootstrapped 5% (conn): Connecting to a relay Jun 13 00:10:24.247 [notice] Bootstrapped 10% (conn_done): Connected to a relay
That is, it's trying to connect to an ORPort on 126.96.36.199:443, presumably to fetch a bridge descriptor. Of course that isn't working because that's an obfs4 port.
Should Tor not count bridges for transports it doesn't have a ClientTransportPlugin line for, and if it ends up with no bridges that it knows how to use, refuse that configuration in the same way that it does now for no bridges?