A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

in extremes • in extremis
in extremes —
1
. is used of extreme magnitudes or conditions:
In extremes, there are micro-skirts, which barely cover the
buttocks; minis, maxis and the nineteen-thirties length. 2. in
critical circumstances: There’s no saying what any man may
do in extremes.
Note: 
a) The expression is not equivalent in meaning to the
phrase in the extreme—(used in postposition) to the highest
degree; to the utmost: I always thought the suggestion dangerous
in the extreme.
b) The expression does not correlate in meaning with the
phrase to extremes—to excess; out of all proportion: Surely we
can have clean air and water without carrying any policy to
extremes?
in extremis —(Latin) 
1
. (dated) at the point of death: The
Master of the Rolls … tumbled out of his chair last Sunday
at church, and is, they say, in extremis. 2. (formal) in very great
difficulties: To use this refuge for ordinary mountaineering is
an abuse; it is only intended and equipped for special parties
in extremis.
in faith • in good faith
in faith —having faith as the basis of a belief: We cannot
prove that there is life beyond death; we can only accept it in
faith.
in good faith —with honest belief or intention: Though his
statement proved to be untrue, it was made in good faith.
in favor of someone • in favor with someone
in favor of someone —
1
. in defense or support of a person:
Let each member speak in favor of the candidate of his or her
choice. 2. to the advantage of a person or team: The defense
players try to keep the score constant, and the forwards try to
change it in favor of their team.
in favor with someone —liked by a person; having smb.’s sup-
port or approval: You can’t expect to be in favor with Sue if
you’re always criticizing her.