This is very common. I guess “I had drank a few beers, but I wasn’t drunk” sounds better than the correct version.
Also, had swam, had ran, had saw, had ate etc., etc.
Wait… so it isn’t “swimmeded”?
Well of course not, because the OED would never, ever split an infinitive. 
Sorry to burst your bubble, but not only do I write, I <gasp> edit other people’s work. I am the final gatekeeper.
Spelling questions frequently come up, but nobody ever worries about grammar unless English is their second language.
This may sound sarcastic, but it’s not: what an anomalously sensible perspective.
You’re not bursting my bubble. You must have a very good intuitive grasp of English grammar. Some people just have a very good ear for the language–sounds like you’re one of them.
It’s possible. But all editors for the NYT have the same in house style manual which they must be familiar with. Some people use the Times style manual as a basic guide to writing style in publishing in general.
Knowing better doesn’t keep people from making mistakes.
Nice combination of user name and post content.
I visualized the poster as a muscular half-naked giant with an axe, whispering “I don’t even know what a past participle is”, while standing in some refined old world library.
OK, while it’s refreshing to have people paying attention to my posts for a change, I should probably stop posting because people can’t tell when I’m being snarky and when I’m just plain confused. I’d honestly never heard of ‘swum’ before now, and had I come upon it in any other context I would have put it down to an author or an editor getting a bit giggly on champagne.
Airman Doors, USAF: Who defines correct usage? You? The small, profit-driven pack behind the OED? The equally small and profit-driven pack behind the AHD? Your third grade teacher who went on a tear every time someone split an infinitive or used a double negative or dared utter a word that Does Not Exist In The English Language?*
*(I have nothing but respect for teachers who know what they are teaching and, hell, anyone who can stand up to a pack of half-civilized yard apes and go through arithmetic for the twentieth time. I have no respect for teachers who abuse their position to spread dogma and lack respect for honest curiosity.)
Correct usage is defined by the authors people want to read, both past and present. Chaucer, for all his glory and joy and for all the wonder of his language to the trained ear, does not fit that and hasn’t for a very long time. Shakespeare still fits to a certain extent, but, i’ faith, his japes and jests fall further from the firmament, even when they descend to country matters. Mark Twain didn’t sound like Shakespeare, and not because he tried to and failed. He used the written language as the handmaiden of the spoken language and succeeded. His innovations are celebrated to this day not to spite the arbitrary rules, but because language is art and its usage is defined by successful art. Superstitious rules based on what some forgotten great grand uncle thought sounded stilted or formal have no place in the language I want to speak.
Finally, bringing up grammar is a complete red herring. Grammar changes at a glacial pace and is in absolutely no danger. English still has the mostly SVO and analytic grammar it’s had since it arose from Anglo-Saxon (once called Old English) after 1066.