Steven has a post about a cliche anime gag that involves a commonly recurring washpan that bonks characters on the head. I hope Steven will pardon me for poaching one of his screenshots. He is an example from Hanaukyo Maid Team La Verite, which I’ve never seen:

This caught my eye because I’ve never seen it before. When people talk about overused anime gags that I’ve never even seen once, it reminds of just how little anime I’ve seen!
A couple of hours after reading that I sat down and watched Full Metal Panic, Disc 4. Well! Lookie what we got here:

DONK!
There it is! Same color. Same Size. Different show. Same washpan. I’m glad I’d just read Steven’s post, so I knew this was a running gag and not random wierdness.
Kaname is bellyaching (again!) about the fact that instead of going to the beach they have come to an AS (giant robot) tournament. Then Kaname’s friend Mizuki points out how it is her own fault she’s here. She chose to come. The moment of realization hits her, and to drive the point home the joke washpan falls out of hammerspace and donks her on the head.

A second one! I didn’t see that coming!
Mizuki heaps shame onto crybaby Kaname, and the washpans continue to pile up. I like how it’s obvious that Kyouko (left) can see the washpans.

Okay, the joke is old.
Small wonder how the joke got worn out, really…
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It’d be interesting to know the source of the gag. But we’ll probably never find out. I suspect this is the Japanese equivalent of a “pie in the face”, a comic cliche so old and widespread that no one really knows the original source.
Second season of Ah! My Goddess!, I think Peorth gets nailed by one. I’m trying to remember all the places I’ve seen it; certainly the most memorable was HMTLV. I’d actually forgotten about that incident; the one I remembered best was in the contest between Mariel and Ryuko. They have a Best of 10, er, 100, er, 1000 (Ryuko keeps extending the challenge as she loses badly at first). In the “washpan catching challenge” she rants and leaps wildly about, expounding on the need to keep one’s eyes open and expect anything, etc., etc. Of course she gets knocked cold as the washpan hits her in the head. A second later, you can hear another one falling, but Marial barely moves –she simply takes one step backwards and catches it about waist high. Very understated — she never changes expression in the entire scene. Which only makes it funnier and makes Ryuko look worse, of course. Not tha Mariel was trying to do that — she was just being herself. Which makes it funnier yet.
I still want the triplets, dammit.
Hm. Nope, apparently there was no washpan or it happened to someone else in the series. Just checked the episode I thought it was in.
But why are they yellow? The yellow seems to be an important attribute. Are they bronze? Tarnished?
Actually, the one that shows up in “Happy Lesson” isn’t yellow. It’s gray/silvery.
Kids used to prop them over doorways to fall on people, especially in school. It’s a moldy old gag, like our banana peel joke, but I’m assured it was really done, back on the 50s and 60s.
In Final Fantasy X-2, there is a dressphere (character class/job) that has access to “Reels” moves, similar to a slot machine, except you stop each reel’s spinning individually. If you fail to get any actual result (I think the most basic is a single cherry icon), then “Dud” is activated, where a washpan falls onto the character’s head and lops off 75% of her HP (I wanna say current, rather than max).
Looping back, a Lady-Luck-looking rival character, Leblanc, often used the word “dud” in relation to the main characters, so maybe this is an in-joke… *shrugs*
EDIT: Good to have some discourse on its running-gag-ness; got here by discerning a word-tag for http://danbooru.donmai.us/post/show/436853/.
The anime version (can’t speak for the manga) of Tsukuyomi Moon Phase is filled with this gag, even using it as a weapon in some cases.