i've always thought that when you do this you're supposed to leave your # on your dash in case the person you're blocking in has to leave. at least, i always do this.
now blocked in, can't leave. and parked what i thought was xx feet from a hydrant so this specifically couldn't happen, and someone scunched their car in behind mine.
(putting aside the fact that not getting tickets for this is a "courtesy" from the police in the first place)