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

drop in • fall in
drop in —
1
. collapse inwards; give way: The roof of the tun-
nel hasn’t been properly supported; it shows signs of dropping
in. 2. (coll.) visit a person casually: He usually drops in at my
place on his way home.
fall in —
1
. fall accidentally into smth.: The child was near
the river and I was frightened he would fall in. 2. = drop in 
1
:
They are afraid that the whole building will fall in on them.
3. (Military) form into ranks: At first light Chelmsford ordered
his men to fall in and the column marched away. 4. (legal) ex-
pire: As the leases of these old properties fall in, they are being
pulled down to make way for office blocks. 5. agree: Once the
chairman had stated his decision, the rest of the committee
fell in.
drop in on someone • drop on someone
drop in on someone —(coll.) visit a person, usually without
having arranged it before: I dropped in on her for coffee the
other morning.
drop on someone —(coll.) reprimand or punish a person:
You’ll have to mind your p’s and q’s or else you’ll be dropped
on.
Note: 
The expression does not correlate in meaning with the
phrase fall on someone—
1
. attack a person: They fell on me
like an avalanche with teeth and dagger, and hatred without
check or limits of sanity. 2. used of a cost, responsibility, etc. to
be borne by a person: So it fell on me to do all the packing,
making sure we had all the necessary equipment.
drop into someone’s lap • land in someone’s lap
drop into someone’s lap —(also: fall into someone’s lap) be
achieved or obtained without any effort: It would not be safe
to assume that victory will drop into our lap at the next elec-
tion.
land in someone’s lap —become an unwelcome responsibility:
These problems have landed in the lap of Donald Jackson, an
unassuming manager with little international experience.