Ahem

We all get a little forgetful sometimes. When things in a plot get really, really complicated, it gets tough to keep track of all the disparate elements. So while Alice should have the sense to search the room for Bob before she starts badmouthing him, hey, nobody's perfect. As a result, she might need a little help from one of her friends. A simple cough, maybe a cold, paralyzing glare, and, Ahem, Bob's kind of standing right behind you.

Who provides the Ahem could be anyone, actually. Bob himself might do it, creating a potentially very embarrassing situation for Alice. The explanation also differs. A character who's too much of a Cloudcuckoolander will need it explained, explicitly, why it is they're saying something stupid, ignorant, and in direct contradiction of the facts. A person of normal intelligence is usually Genre Savvy enough to immediately go back over their words and realize what the massive intuitive flaw in their previous statement was.

Examples of Ahem include:

Film

Seen in Austin Powers:

Dr Evil: ... unless the world pays us a hefty ransom!
Number 2: (coughs) That already happened.

Jeff Goldblum's rabbi dad doing this in Independence Day to the President. Multiple times.
In the 2005 King Kong movie Ann mistook someone for Jack and was babbling about how he didn't fit the stereotype she'd been expecting, when the real Jack snapped his book shut behind her; she turned to see that he was the exact stereotype she'd been describing. Not awkward at all.

Literature

Umbridge in Harry Potter. "Hem Hem". Interesting in this case because the points she makes are rarely obvious—often she has to do it a couple of times before anyone will pay attention to her. Partly because it sounds like a cough, partly because none of the teachers will pay attention to her if they can avoid it. This is obviously to her advantage, though; she uses it to slowly kill whatever conversation was taking place and then holds the floor. One teacher ignores her after multiple coughs and then finally offers her a cough drop.

Live Action TV

Seen in Friends:

Monica: No, look, she's obviously unstable, okay? I mean she's thinking about running out on her wedding day.
(Rachel slowly turns and glares at her.)
Okay, fine! But I mean, look at the position she's putting him in! What's he gonna do? Ross is gonna run over there on the wedding day and break up the marriage?! I mean, who would do that?!
(Rachel again turns and glares at her in disgust.)
Okay, fine, all right, but that's y'know, it's different! Although it did involve a lot of the same people.

Jon Stewart, doing a silly impression of Sam Donaldson when Peter Jennings 'ahem's him that Donaldson is there.
From British show Misfits:

Nathan: I mean we did sod all with our powers but at least we never abused them. We never raped or murdered anyone.
(Everyone looks at him.)
Curtis: *points at Alisha* She raped me! And we killed loads of people.
Nathan: Yeah, but...we're the good guys.

In fairness to Nathan, the others had probably kept the rape incident to themselves up until then. Also, as for the people they killed, those wouldn't fit the category of murder because they were in self-defence.

Gary in Men Behaving Badly series 4, after being dumped by Dorothy:

Tony (on phone): Gary's been behaving like an arse!
Gary: Ahem...
Tony: I mean a nice arse!

Video Games

In the first Discworld Adventure Game, Rincewind discovers the dragon's hoard and begins celebrating, saying that the riches are his and no big ugly scaly dragon is going to stop him. A deep "Ahem" and a quick pan of the camera shows the dragon is right there, causing Rincewind to try and backtrack hurriedly.
The spy from Team Fortress 2 at the beginning of the "Meet The Spy" Video when the briefcase is found. Followed by "Gentlemen?"
Done a few times throughout Tales of Monkey Island. There is one part in Chapter 5 when, after Guybrush thanks Morgan for opening the Crossroads, she gives out an "ahem", which, in the subtitles, is stated as "*clears throat*".

Comics

In a Star Wars comic, the group were talking about the Death Star's port opening. Tarkin said that no one could make that shot. Then, Darth Vader did the ahem right behind him.

Web Comics

Dan and Mab's Furry Adventures, Biggs ordered his Dragon, Dee, to "dispose of" Alexsi. Alexsi is Dee's (adopted) daughter.

Dee: *Ahem*
Biggs: Hmmm? Oh, yeah! The daughter thing. Right. My bad there. Well, then! I guess you're free to go! Have a nice day!

Last panel of this page of Girl Genius (2008 Xmas break story)
The Inexplicable Adventures of Bob has a Starfish Alien named "Ahem," whose name is pronounced like someone clearing his throat.
In Gunnerkrigg Court, Reynardine The Great is caught playing with dolls by Annie, as she comes up behind him with a quiet "Ahem". Here.

Western Animation

Xiaolin Showdown
- Omi is picked to stay behind from a Shen-Gong-Wu search to watch Dojo:

Omi: What!? Why must I stay and Raimundo go? He was the last to make it to apprentice, and nobody thought he would make it!
Raimundo: AHEM.
Omi: ...Uh- er- that is...except for me?

- Omi explicitly states "You know, I'm standing right here."
After discovering that his brother-in-law's girlfriend was an employee of one of the villains, Doc says to him:

Doc: I'm sorry about Abbey.
Doyle: Thanks.
Doc: I mean, if anybody in that relationship was gonna be secretly evil, I'd have put my money on-
(his wife pokes him)
Doc: (clears throat) Well, we'll leave it at 'sorry'.

Averted in Futurama, in the episode "How Hermes Requisitioned His Groove Back", with Bender going to the rest of the crew about Fry sleeping with Morgan Proctor, and Amy *hem hem*s.
- Bender: *reaching behind him* "Oh, crap, is she behind me?"
- Morgan Proctor: "No, I'm in front of you."