Visible Boom Mic

Oh, I'm being followed by a boom shadow...

Whoop, boom mike, big twelve.

—Joel Robinson, pointing out a VBM in Mitchell

And now you can see his head? The boom mike is so far into the shot I could describe the boom mike operator to a sketch artist!

—The Cinema Snob on Dolemite

When a boom microphone or its shadow makes it into the final production by accident, it detracts from the show by reminding the audience it's just a show. This may be done on purpose for parody or to break the fourth wall.

This is a common Blooper in old Soap Operas, Sit Coms and dramas videotaped (sometimes filmed) with Three Cameras, where all of the dialogue has to be recorded using overhead boom microphones. Less common on single-camera TV shows and movies, but it does happen. Much more scarce today because drama shows don't use multi-camera any more, and any stray shadows can be CGI'd out.

Examples of Visible Boom Mic include:

Anime

Whenever the Sinnoh Now! crew shows up on Pokémon, expect someone to be clobbered by the boom mic at one point.
Used intentionally in a trailer for the Makai Senki Disgaea anime.

Film

One of the most well-known Special Effects Failures of Plan 9 from Outer Space is this, during the cockpit scene.
Parodied in The Kentucky Fried Movie, where the boom mic descends so low into the frame that it actually starts bumping into the actors.
Used purposefully in the movie of the musical Hairspray, with a period 1960s boom mic.
Ransom, the Mel Gibson movie: One can be seen just after Gibson realises his kid's missing.
Blatant in soap opera spoof Young Doctors in Love.
Used purposefully in Black Dynamite.
Happens so often in Dolemite that The Cinema Snob made a drinking game of it.
Happens once in Incident At Loch Ness. It's not clear if it was intentional or a happy accident, though; the movie's a Mockumentary, so it's easy to justify it as being part of the film crew.
A boom mic appears under Steve Zahn in an early scene in Strange Wilderness. Whether it was intentional or done on accident, this shouldn't be happening on a film that was filmed on anamorphic lenses.
For one shot late into Steel Dawn, while the boom mic itself isn't visible, the director did make the mistake of shooting outdoors with the sun behind the camera, so that the operator's whole shadow falls right across the scene.
Visible in 'Blazing Saddles' as well, on the bottom right side once.
Several scenes in the film Hop.
The Uma Thurman / Meryl Streep film Prime is filled with these. You could easily make a drinking game out of spotting the boom mics.

Live-Action TV

The Twilight Zone TOS episode "Will the Real Martian Please Stand Up": During the final scene between the businessman (three-armed Martian) and the counterman (three-eyed Venusian), as the counterman says, "And I agree with you about what they call music," a shadow from an overhead boom mike (on the counterman's hat) moves slightly.
The information text on Doctor Who DVDs helpfully points out the more prominent boom shadows as well as other technical flaws.
The directors' commentaries on the Fawlty Towers' DVD set also point out some of the worst boom shadows.
In the first episode of St. Elsewhere, there's a long dialogue scene where the characters are walking through the corridors, and the mike is visible for a few moments.
One of the worst offenders is the original Beverly Hills, 90210 series, where the Visible Boom Mic could be spotted countless times floating over a main character's head.
This happens all the time in Red Dwarf. There are even compilations on YouTube of shots with visible boom mikes, although the Boom Mikes themselves, even when visible, usually blended pretty well.
Happens in Corner Gas when Brent and Wanda are discussing the comparative qualities between production values and substance.
- To clarify: The boom mic drops down and hits Wanda in the head, she then grabs it and throws it back off screen.
This happened so often on South of Nowhere that the fan forum declared Boomy the Boom Mic an integral character on the show and came up with a variety of Epileptic Trees and Parody Fan Fics to tie Boomy to the plot.
An interesting is example is in the "Breaking Glass" episode of MythBusters. Since this is a science reality show, the visible boom mike isn't a problem as far as suspension of disbelief. The problem is that in the scene in which singer Jaime Vendera shatters a wine glass with only his voice, some viewers thought that the visible boom mike was some sort of sonic gun used to break the glass. Incidentally, in the same scene, some viewers thought a lens flare was a string attached to the glass, somehow used to shatter it. Both can be seen here starting at the 0:43 mark.
Deliberately used in the monologue of a Saturday Night Live episode hosted by John Lithgow.
The boom mike showed up in the shot at least once during the 10-year run of Friends.
Accidentally averted in Charlie Brooker's Screenwipe. One episode purports to demonstrate the various problems with filming on location - batteries running out on the camera, keeping anything unwanted out of shot, and so on - and one of the things they meant to show was letting the boom mike fall into shot. Except they forgot to lower the boom mike, so Charlie explains that in real post production they had to greenscreen in a boom mike just so that in fake post production they could cut it back out.
Played for laughs in Arrested Development when the Bluths discuss whether Michael's British girlfriend might be a spy trying to eavesdrop on their business plans. As Michael argues that nobody is trying to listen in on them, the board room's boom mike is clearly visible.

Web Animation

Referenced in a Homestar Runner marshmallow commercial. Homestar falls asleep during filming, on camera, and someone has to tap his head with a boom mic to wake him up.

Web Original

In the 6th episode of the TV Tropes webseries Echo Chamber, the boom operator, cameraman, and boom are visible in the window reflection. However, the cameraman, Zack, was already a major character on the show, and Word of God has hinted that the boom operator is his brother, Dave.

Western Animation

In The Simpsons episode parodying The X-Files, Kent Brockman mentioned that the boom mike operator was being fired the next day, then the boom operator hit Kent in the head with the mike.
The Angry Beavers episode "The Loogie Hawk"
Used in a Plan 9 parody on Mission Hill.
Happens on the Show Within a Show in Bolt. The director is angry about it, not because the audience could have seen it, but because the dog could have seen it, since it would have spoiled his Enforced Method Acting.