I'm just curious, those of you whom English is a second (or third, fourth, whatever) language, how prevalent is grammar mistakes in your native language, how is your language structured, and if it's common to see grammar mistakes in your language amongst native speakers. With English being one of the most common languages in the world, approximately 1.8 billion people can speak English to at least a reasonable degree, however, less than half of those are native English speakers (ie, as their first language).
As a native English speaker, I can't help but notice that many other native English speakers can't even grasp some of the most basic aspects of the language, case and punctuation. It's strangely common to see incorrect spacing around commas (it's always comma-space, the space is not optional). Likewise, sentences starting with lowercase characters, the absence of commas when they're necessary (let's eat, grandpa; let's eat grandpa), and commonly mixed up words (there, their, they're).
Obviously nobodies expecting non-native English speakers to have flawless English, although it makes me flinch at how often even native speakers who graduated from high school with mandatory English classes can make such simple mistakes. What common language-related flaws are prevalent in your native language (who knows, perhaps it'll keep us non-native speakers from making them)?