The American past participle of get, gotten, is one of those American things that the British often express real distaste for. I get the feeling that some Brits think it sounds ignorant. Better Half is now shouting from the other room that it sounds uneducated and hillbillyish. It's an example Americans keeping an older form that disappeared in Britain. A lot has been written on this subject. I recommend the following:
For a bit more on the history, see Maven's Word of the Day.
See John Lawler (with help from David Crystal) on why British people often get it wrong when they try to use American gotten. Essentially, with the 'possession', rather than 'acquisition', sense of get, we say have got, not have gotten. This means that the following two sentences mean different things.
I've got a new hat. (= 'I have a new hat.')
I've gotten a new hat. (= 'I obtained a new hat.')
The thing that I find a bit funny about the looking-down-the-nose attitude toward gotten is that it's retained in British English in the participial verb forgotten (hardly an uncommon verb!) and in the adjective ill-gotten.
Americans also have an irregular past/past participle for fit, but this one isn't so old.
US: Before he lost weight, the jacket (had) fit him.
UK: Before he lost weight, the jacket (had) fitted him.
In my dialect (or at least my idiolect!), we do use fitted when describing making something to measure. So:
US & UK: I had that jacket fitted. The tailor fitted me for a jacket.
But according to The Oxford Dictionary of American Usage and Style by Bryan A. Garner, I'm part of a dying breed and others are using only fit as the past tense of fit:
"Just since the mid-20th century, AmE has witnessed a shift in the past tense and past participle from fitted to fit. Traditionally, fit would have been considered incorrect, but it began appearing in journalism and even scholarly writing as early as the 1950s.
The traditionally correct past tense still surfaces—especially in BrE—but in AmE it is becoming rarer (and stuffier) year by year: “A most interesting item in my coin collection is a disk that fitted the pressure-spray nozzle on our apple-orchard pump some 50 years ago” (Christian Science Monitor). Although fitted may one day be extinct as a verb form, it will undoubtedly persist as an adjective fitted sheets."
Presumably the irregulari(s/z)ation of fit is on analogy with hit, which does not change its form in the past or past participle in either dialect.
Incidentally, if a tailor makes you a suit in the UK, it's said to be a bespoke suit. In the US we'd say tailored or made-to-measure, which is perfectly sayable in the UK too. Anything that's made to personal specifications can be bespoke. Checking the web, I got "bespoke vehicles", "bespoke network solutions", "bespoke mirrors", "bespoke browbands" (for horses).
Even more incidentally: Fit is also a recent BrE slang adjective meaning 'attractive'. Of course, this is more related to the 'fitness' sense of fit. I have no idea whether this has currency in the US now--I have heard it there in a British song: "Fit but you know it" by The Streets, which is full of lots of other Briticisms, which I might get around to discussing some day. Right now I'm being amused by a new antonym pairing: fit/fat.
Enough with the incidentals.
Lately, I've been losing my intuitions when it comes to knit versus knitted. According to the New Oxford American Dictionary, the past tense knit is most likely to be used to refer to the process of making rows of looped-together yarn (or wool as is more commonly said in the UK), and less likely to be used in other senses, like making a whole garment or 'knitting' your brow. According to the aforementioned style guide, past tense knit has taken over. As far as I can tell, I say knit for all but the figurative senses. So, I'd say: Celeste knitted her brow while she knit her scarf.