Technically not an error but still annoying is to append an apostrophe and an s to a name to indicate the genitive.

Even technically I’d consider it an error - the genitive/“possessive” apostrophe in English highlights that you’re dealing with a clitic, attached to the end of the noun phrase; e.g. the dog**'s** food` → the dog and the cat**'s** food. In German however it doesn’t behave like a clitic, it’s a plain declension; e.g. das Futter des Hundes → das Futter des Hundes und der Katze - you’re switching words, not moving them.

I wonder if that’s because most people nowadays use von+Dative instead.