Ann Burlingham wrote (somewhat edited here):
The first time I know of that I’ve come across (ahem) the word ”acrossed”, and I only noticed it the second or third time I read a friend’s Facebook status – it completely slipped past my eyes yesterday and earlier today: “off to make the treacherous path acrossed 39 from Castile to Perry [two towns in upstate New York], wish me luck!”
Now the form in question is (in phonemic transcription) the non-standard variant /əkrɔst/ (the phonetic value of the /ɔ/ varies from dialect to dialect), corresponding to standard /əkrɔs/, spelled ACROSS.
It’s well-documented as a U.S. variant associated largely with “less-educated” speakers in many parts of the country (though non-linguists tend to report it as peculiar to wherever they happen to live — because, of course, that’s where they notice it). It’s covered in DARE, under the spellings ACROSST and ACROST, where it’s connected to the non-standard ONCET for ONCE, and was discussed last year in Language Log by Mark Liberman: “Ask Language Log: “acrosst” ” (here).
Peevers are infuriated by the variant, especially in speech. An Urban Dictionary entry for acrossed pronounces it an “ignorant mispronounciation [sic]“. Elsewhere you can find the inane “not a word!” judgment that’s applied to variants that the peever dislikes:
acrossed is not a word
acrosst: no such word
acrost: An urban legend. A myth. It doesn’t exist.
(For a recent attack on this way of thinking, see Mark Peters’s column “Let’s Bury the Not-a-Word Myth”. Others have gone before him: Erin McKean (who he cites), Eugene Volokh, and me, among them.)
ACROSSED is mostly cited from writing (as in Paul Brians’s Common Errors), where it’s frequent, ACROSST and ACROST mostly from speech, though those spellings occur as well as ACROSSED:
i was hit as i walked acrossed a parking lot by a car. i did not have any broken bones but have soft tissue injury what do i do (link)
First time I’ve come acrossed this one. (link)
I just drove acrosst the Mackinac Bridge… (link)
Sliding The Blade Acrost My Skin [from a cutter] (link)
The spelling ACROSSED models the spelling ED for pasts/past participles in /t/, as in CROSSED (and compare PASSED and PAST, which share a history); the spelling makes some demi-eggcornish sense out of /əkrɔst/.
It’s significant that across and once both ended in /s/, so that acrosst and oncet could be understood as actually having the -st of amongst (~among), amidst (~amid), and whilst (~while) (and also, historically, against and betwtixt). These three items have an interesting history: the older variants were amongs, amids, and whiles, with what was originally the -es of the adverbial genitive; and they then, according to the OED, picked up the -t as a result of being “corrupted” by association with the -st of the superlative.
These other -st variants became standard, while acrosst and oncet seem to have gotten left behind.
[Extra note: the old adverbial genitive survives in besides (alongside beside) and towards (alongside toward) and, in fact, whiles (as a variant of while), plus a collection of other items: always, nowadays, inwards, outwards, backwards, forwards, sideways, etc. And two deprecated non-standard variants: anyways 'anyway' and aways 'much, far, way'.
And no, the all(')s of all(')s I know is is something else again (the s represents a complementizer use of as). And there are still other sources for final s.]