In the film version of "The Two Towers" the preparation for the defense of the Hornburg has a company of elves show up, claiming to have
been sent by Elrond to the defense of Rohan.
The trouble with
this is that these elves are clearly from Lothlorien, therefore not
under Elrond's command in any way unless Haldir changed his allegiance
in the few days since the end of the last movie. Lothlorien is 200 miles
away over ground that no infantry army of any size could get through
without a supply train. Supposedly Elrond contacted them after the
cavalry skirmish against the wargs, which happened that very day.
So either Peter Jackson made a lot of silly mistakes just to get elven
archers on screen, or Haldir's statement is, to put it politely, a
baldfaced lie. It would make far more sense if the elves had been a light force of scouts
sneaking around the Deeping Coomb for their own purposes, and then got spotted by
the Rohirrim and had to come up with a hasty explanation.
I imagine the scene.
"Ho there! What business have elves among the Riddermark? And armed, no less!"
"Crap. What do we do?"
"You just let me do the talking. Uh yes, we...bring word
from Elrond of Rivendell."
"Dude we're clearly not Rivendell elves."
"You expect this guy to know the difference?"
"I expect him to know that Rivendell is hundreds of leagues
from here!"
"Whatever. I don't want to make Galadriel look bad. An
alliance once existed between elves and men. Long ago we fought and died
together. We come to honor that allegiance."
"It's 'alliance' not 'allegiance' you idiot! Now he thinks we've
sworn fealty to him!"
"Brandir, shut up."
"Elrond of Rivendell, eh? It seems elves march at a
pace faster than any man. Perhaps I shall have to ask Elrond to send me more of
his forces tomorrow."
"Shit. I think he's on to us."
"I think he cottoned on to you'ns as soon as you showed up.
Didja think these Men ain't heerd tell of the Eldar?"
"Shit, they've got an Elf. What the hell is a Greenwood Elf
doing here?"
"The king is also rather in need of battle strength, my
friends. You should not have felt the need to lie to him."
"How the hell does a Man speak Sindarin?"
"By grow tall on Imladris, Grandsirs."
"How the fuck is a Dwarf speaking Sindarin?"
"Guys, that's Elessar of Imladris. Good to see you Elessar!
Do you mind translating for us, my Westron is clearly rusty."
"Just as soon as I know why you have chosen to skulk here."
"Hey, your boss is Elrond, right? I don't have to tell you
anything."
"Perhaps not. But if my lord is Elrond, as you say, my honor bids me tell him that a company of elves falsely swore their
allegiance to Imladris. You will tell me why you have come."
"Galadriel wanted mushrooms."
"I am not easily fooled either, Haldir."
"Stroll in the meadow?"
"Thrice I will ask, and no more."
"Okay fine! Galadriel wanted us to pick off the Dundenling
scouts at the other end of this valley and she didn't want it to look like she
was sending an armed company against Saruman openly until it was time to strike
and she didn't want us to get spotted by Men because the Rohirrim are twitchy
about foreigners when they're not on the main roads. Are you happy now? Can we
go?"
"I did tell you the king needs all the battle-strength he
can find, my friend. You have done your work at one end of the valley, but now
come the Uruks against this castle. I would that you stand with us."
"Saruman jumped the gun, eh? Alright guys, we've got more
work to do. King of Rohan! We are proud to fight alongside men once more. Where would you have us stood?"
"Above the gate house."
"Shit."