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

fall off a truck • fall off the turnip truck
fall off a truck —(U.S. euph.) said of an item that was prob-
ably stolen or obtained by questionable methods (also UK: fall
off the back of a lorry): It shouldn’t be surprising that in such
circumstances a lot of those products will “get lost in ware-
houses” or “fall off a truck.”
Note: 
The expression does not correlate in meaning with the
phrase fall off the wagon— resume drinking after having
stopped: Broderson fell off the wagon again. After a week of
heavy drinking, he pawned the trailer to an acquaintance.
fall off the turnip truck —(U.S. coll., usually negat.) be naive
and gullible: I spent almost ten years in the private sector, so
it’s not like I just fell off the turnip truck.
fall out of something • fall outside something
fall out of something —
1
. drop out of smth.: Suddenly, some-
thing fell out of the Bible. What he saw was an old leaf that
had been pressed in between the pages. 2. (used figuratively in
phrases such as “fall out of love,” “fall out of use,” etc.): It is
interesting how words fall out of favor or dramatically change
in meaning over time.
fall outside something —be beyond the scope of one’s respon-
sibility, interests, etc.: What they are asking me to do falls
outside my technical skill set. Cf.: fall within something — be
part of one’s responsibility, interests, etc.: I am open for almost
any type of work that falls within my expertise.
fall through the cracks • fall through the floor
fall through the cracks —(also: fall between the cracks) pass
unnoticed; be neglected and not dealt with: In the past, many
learning disabled children fell through the cracks.
fall through the floor —(also: sink through the floor) 
1
. be very
surprised: The Security chaps nearly fell through the floor
when I walked into Parliament so early, so goodness knows
what they were imagining. 2. suffer extreme embarrassment:
We stood there with our hands crossed over our crotches in
misery. I was about ready to fall through the floor with shame.
3. (of prices, sales, etc.) reach the lowest point: While some
vegetable prices have fallen through the floor, domestic prices
for crops are markedly higher than abroad. Cf.: hit the ceil-
ing —(of prices, sales, etc.) increase very rapidly; surpass the
expected limit (also: hit the roof ): The taxi drivers were aware
that the buses were not running on this particular day, and
prices hit the ceiling.