What a hilarious thread.
I work mostly with people whose first language isn't English, so they have a good reason for not getting things 100% right, but I find it quite interesting what people consistently get wrong.
Correct verb conjugations are tough. I see the right verb in the wrong tense a lot. Not really complex things, either, but present when they meant past or plural when they meant singular.
Prepositions are also difficult to use correctly. Some are straightforward and tangible--the cat is ON the box, the cat is IN the box--and others are kind of abstract and dependent on memorization more than definition--do you conduct research ON a subject or IN a subject or OF a subject or FOR a subject?
In Korean they don't have articles (a/an/the), so my Korean co-worker tends to simply leave them out. This drives me crazy when I proofread his stuff, because there are SO MANY of them. I told him to at least try to put them in, he ought to get some right just by chance if nothing else, and then I wouldn't have to mark them all.
I had one co-worker who kept capitalizing random words in a sentence. I don't know where that came from. Her native language was Spanish, not German, where I understand they capitalize a lot of nouns, but I don't think they do that in Spanish.
Word connotations are really interesting. A lot of people look up the word they want in a native language-to-English dictionary, but you can miss a lot of nuances that way. One co-worker wrote a protocol in which he repeatedly used the word "grab," as in "grab the ethanol" and "grab the beaker" and "grab the hot plate." He meant "get" or "obtain" or "go over to" or "use" or various other things. My boss freaked out and made him rewrite it. It does sound rather informal. But, she also had visions of people "grabbing" like parents grabbing for the last Tickle Me Elmo at the toy store on Christmas Eve--violently and without care. Because apparently we're all morons who would take the instructions literally.
My Spanish-speaking co-worker once sent me an email asking me to help her "redact" her report. That immediately brought of images of the military blacking out sensitive information before they release documents, so I was like, wah? I actually looked up the word and some dictionaries list it as a synonym for "edit," which was what she was really going for, as in "proofread." Connotations...