I learned Dutch before I started learning German (having lived in The Netherlands for almost a decade) and they're quite close as languages go (at least for somebody whose mother-tongue is a romance language) so that was pretty useful, but the one thing that really got me a lot in the beginning is that in German, "wie" means "how" but in Dutch "wie" means "who" (and both words sound exactly the same), so I would hear the very common German greeting "wie geht's" (how's it going) and would translate it as "who goes", and even after knowing the meaning properly it would trip me since the mental "circuitry" doing the translation seemed to be the instinctive one I had developed for Dutch.