Learning from your mistakes is the most painful and the most efficient way to learn.
Learning from other people's mistakes is way more entertaining.
For example, when playing card games with Germans, in English, I noticed that the Germans "mix" the cards and "give" them (instead of shuffling and dealing them). So it must be "die Karten mischen" and "die Karten geben" in German. In trick games, you might hear Germans saying "stitch" instead of "trick", because it's "Stich" in German.
In the morning, German students stand up and then they learn the whole day, so, aufstehen = get up, and lernen = study. As a hobby, they might "make photos" instead of taking them ("ein Foto machen"). By the way, Hungarians do the same. And they cannot distinguish between "come" and "go". This will be helpful if I ever want to learn Hungarian.
I was chatting with a German friend once, and he used the word "overhear" in a way that didn't really fit the context. I figured that "überhören" must be a German word, checked it from dict.leo.org, and indeed, it means "to miss", in the sense of "to ignore a remark", "eine Bemerkung überhören". The same happened with another friend and "überlesen" vs. "overread". For the record, overhear is "(zufällig) mithören", so if I ever (over)hear somebody saying "withhear", I know what they mean.
Germans also say "to" when they mean "too" (to run, too difficult). It just dawned on me, that in German they actually are the same word. ("It's too difficult to run" - "Es ist zu schwierig zu rennen.")
Not all mistakes are as enlightening though. For example, Germans often mix "some" and "any", for example say "I didn't do something" when they mean "I didn't do anything". I don't yet have any insights (or should I say "some insights") into why they do that. Similar constructs exist in German too.
I feel so culturally adjusted when I accidentally say "become" when I mean "get" (in German "bekommen"). Can I get the nationality faster if I make such embarrassing mistakes?
PS. In many cases, the German way of saying things and the Finnish way of saying things are similar, and the English way is different. Like, in English you can say "no idea", but in German you say "keine Ahnung" instead of "keine Idee", and in Finnish too, "ei aavistustakaan" instead of "ei ideaa". It's so annoying when I say things wrong because I try to use the English way when speaking German, whereas I would've got it right by translating directly from my native language.