In my experience, many German native speakers (including myself) tend to mispronounce the noun worm, referring to a long thin creature with no bones or legs, that lives in soil. The mistake usually made is to pronounce worm as [wɔː(r)m], just like the word warm, see here. However, the correct pronunciation of worm is instead given by [wɜː(r)m], with the letter o pronounced just like in word. You can listen to the correct pronunciation here. I don’t have an explanation why I make this mistake, because I never have problems pronouncing the o in word as [ɜː]. But now that you have been made aware of this mistake, I’m quite sure you will notice how often it is made by German native speakers, and possibly by yourself.