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From the excellent TV Tropes Wiki:

Facefault

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When shocked or surprised, often by an absurdity or non sequitur, the listener may fall over onto his face, his limbs in a twisted mass above him. A device usually limited to humorous anime, this may have been imported from American slapstick comedy.

I believe the original was a joke definition post by Martin Rose to rec.arts.anime in 1992:

Face fault : (noun) falling to the ground, landing on your face. Ex: Lum flew through the door and Ataru did a face fault.

The term is fairly common in anime fandom, and I’m mildly curious as to its derivation. “Face” is obvious enough, but the only source I see for “fault” is the incredibly nerdy use of “segfault” as “an exclamation at the point of befuddlement”. It’s amusing if it’s true, given that the popularization of anime in the West over the past few years means that many fans are unlikely to have even heard the word “segfault”, let alone know what it means.

I ran across it in some Ranma ½ fanfiction; it was “the family facefaulted” here and “Ranma facefaulted” there, and even one notable (and anatomically difficult) “facefaulted sideways”. The curious thing is that it’s utterly incomprehensible to anybody unfamiliar with anime, but to fans it’s clear enough — especially in context.

Fanfiction of this sort isn’t a textual representation of fictional events, it’s a textual representation of a cartoon representation of those events. There are various manga tropes — like the facefault — which are convenient symbols that help offset the deficiencies of the medium. When you’re drawing small and not terribly detailed pictures, a character falling on his face is more obvious a symbol of shock than an open mouth or raised eyebrows. Even if these deficiencies don’t translate to a new medium, such as plain text, the symbols get ported across anyway.

One example is the stylized sweat drop used to indicate confusion or embarrassment, which ties into the link between sweat and stress. It’s hard enough to draw sweat on a cartoon character without it looking like running eyeliner; it’s impossible when the character’s head is smaller than your fingertip. A stylized drop solves this problem, with the added benefit that the artist doesn’t need to bother overly with subtle facial expressions. In text form it’s easy enough to write “she sweated” or “he started sweating”, or expand into florid description of crinked brows, quavering lips, slight blushes and other subtle signs. Instead, there’s often a marked lack of descriptive prose in favour of “he sweatdropped” or “a large sweatdrop formed on the back of her head”.