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

fly in the ointment, a • hair in the butter, a
fly in the ointment, a —some small or trifling circumstance
which spoils the enjoyment of smth.: There is only one fly in
the ointment offered by commercial propagandists; they want
your money.
hair in the butter, a —(U.S. sl.) said of a delicate situation
or sensitive issue: Never mind the details — which neither the
Governor’s office nor anyone else is willing to share just yet —
this is a real hair in the butter.
fly off the handle • go off the handle
fly off the handle —(coll.) get into a rage; lose one’s temper:
When we are under stress, we are more likely to fly off the
handle.
go off the handle —(coll.) 
1
. = fly off the handle: Jimmy went
off the handle at once; and Nicol kept patting him on the
back. 2. die: My old gentleman means to be Mayor before he
goes off the handle.
Note: 
Neither expression is antonymous in meaning to the
phrase get a handle on something— understand or be able to
master smth. fully: This book should prove useful for anyone
who wants to get a handle on this complex subject.
fly on the wall • let that fly stick to the wall
fly on the wall —an unperceived observer who is able to over-
hear discussions, etc.: “What did you wheedle out of Maggie?”
“Well … she was no fly on the wall, but there have to be
letters, documents, papers to be typed.”
let that fly stick to the wall —(Scot.) say nothing more on
that subject: We’ve got in our midst a newspaper; some people
call it well, a very peculiar newspaper. But we’ll let that fly
stick to the wall.
fly paper • fly-sheet
fly paper —sticky paper for killing flies: Once the first fly
lands, even more flies are attracted to the fly paper — making
this product one of the most effective ways of dealing with
flying insects.
fly-sheet—
1
. a small loose advertising sheet : Brandt had
received money with which to publish and distribute fly sheets
inciting the unemployed to violence. 2. a flap consisting of a
piece of canvas that can be drawn back to provide entrance
to a tent: A fly-sheet is desirable in heavy rain, but none have
been designed which will stand up to a strong wind.