Mistakes Natives Make

As an English teacher working with non-native speakers, I see more than my share of mistaken grammar, usage, and spelling - but you may be interested to know that native English speakers also make lots of mistakes. The difference is that because native speakers learned their first language in an environment where they use it every day, the types of mistakes they make tend to be more ones of habit and convenience than ones of misunderstanding. In fact, correcting a native speaker's technically mistaken grammar can be a good way to get you called a "pedant", or someone who obsesses over details and academic minutia.
I happen to be quite pedantic when it comes to language, so mistakes natives make concern me quite a bit - even more than mistakes from non-natives, since at least non-natives have a perfectly legitimate excuse not to know how the language works. Here are a few common errors native English speakers make that I rarely if ever see from non-natives:

"Everyone has their own problems"
The word "their" has to refer to two or more people, whereas "everyone" is grammatically singular. Natives quite commonly use "they", "their" and "them" when the subject of their pronoun is of unknown or undetermined gender, even if it is only one person. The correct form, however, is "Everyone has his or her own problems", using singular pronouns.

"Lady's clothing store, its a real bargain"
Grammatically correct use of the apostrophe trips up many people, and turns what used to be a simple printers' mark into a bone of contention among amateur and professional grammarians everywhere. The first word in the above sentence, "Lady's", should be "Ladies' ", since the store is presumably for more than one lady. The word "its", however, needs an apostrophe between the t and s to make it clear that this is an abbreviation of "it is", and not something that belongs to it. Most people can tell what these mean in context though, which is why efforts made to correct them so often fall on deaf ears.

Natives speakers learn their language descriptively, as a tool to explain and communicate, rather than prescriptively, as a system of unbreakable rules. While I believe describing and communicating are the primary functions and in fact the raison d'etre for language's existence itself, rules that sometimes seem arbitrary can say important things to other people who know the same rules - and that's why I try my best to protect them. They are one more tool of communication that I don't want to see dulled.