1) I still am very wary of Trennbarresverbs (verbs with prefixes that can change entirely the meaning of the verb and which sometimes move to the end of the sentence).
Ich bringe dich ... um!
I bring you (somewhere), but with the 'um' it means I'll kill you. (verb: umbringen)
2) Have to make an effort to say Schwiegermutter (mother-in-law) instead of Schwierigmutter (schwierig means difficult) no matter how apt it might be for some.
When living in Sweden, I often felt a bit sorry for the girls who had to live in "Slutgatan" whenever I crossed it.
Slut- end (or a variation of it) gatan -street
As well as the question "Vill du fika med mig?" - "Do you like to have a coffee with me?", let me frequently blush in the first months in Sweden. Sounds too much like.... "You like to f***?" Well, those who understand german will know..
Yeah, I used to trip up on the negations of modal verbs too (Can, Could, May, Might, Must, Ought to, Shall, Should, Will, Would etc) and had to backtrack on the verbs themselves so the nicht-ed form still made sense.
IIRC, a significant number of them with "nicht" changed meaning subtlely but significantly from the English equivalent verb with "not"
So, as you described, I had to relearn muss as "have to", and unlearn the "must" bit and work backwards from the others too - hammers home the subtlety/ambiguity of saying things like "Fred says he may not join us tonight." (Is Fred unsure whether he'll make it, or has Mrs Fred put her foot down?)
Still trip myself up from time to time tho', but I can't grumble, sorry, I mayn't grumble, sorry, I shouldn't gru... oh, never mind!
You offer to top up a swiss person's wine glass and they reply 'danke ich muss noch fahren' which of course actually means 'no' but still somehow sounds like an affirmative answer (occasionally also followed by physically covering the glass with their hand just in case you didn't get the message).
Yes! I've definitely had this situation happen to me, and I've been told that when refusing something, Swiss people always start with the ''Danke'' followed by the ''Nein'' as opposed to the backwards ''No, thank you'' in English. I don't know if the always part is true, but I've definitely gotten some absolutely shocked and dirty looks at stores when saying ''Nein, danke'', as if I had been very impolite.
I'd say "Nein, danke" is more frequent than "Danke, nein," but the important thing in both cases is the "no." However, when there is no "no," as in the case of "Danke, ich muss noch fahren," things can get confusing for those who are not familiar with such peculiarities.
It's not a Swiss or German problem, though. Such misleading situations can happen in just about any language. During my first months in Greece, I had to learn all sorts of such subtleties. Like, when you say, "Den katalaveno," ("I don't understand"), that means you don't understand the language, but when you say, "Den katalava" ("I have not understood" -- yes, sounds silly, but there is no really equivalent translation for the Greek aoristos), it means, "Pardon me?" I learned that during my first hour in Greece.
On our little island, when the proprietor of one of those picturesque taverns on the waterfront asks me, "Do you want to have a beer?", that means he is treating me to it. If I tried to pay afterwards, I'd badly offend him. If, on the other hand, he asks, "What do you want to drink?", he's expecting a normal order.