We only pay attention when we hear mistakes.

Do you mean less attention compared to a native speaker ? No, as the native speaker does pay almost no attention, its (English language) knowledge already moved in a part of a brain that belongs to intuition (hidden or automatic cognitive processes); we foreign learners must pay a lot of attention to grammar and usually get poorer results; we wish that our listeners not only understand the meaning but also correct our errors. Do you frequently consult grammar books for discovering new patterns ?

In my experience if you start paying too much attention to grammar while you're trying to express yourself with someone, you simply end up by going into analysis paralysis. We shouldn't learn HOW a language works, much like we don't need to know HOW a car works to simply drive it. Also we shouldn't be allowed to drive around recklessly when we still have to PAY ATTENTION to all the actions required to drive a car. Language has to become a habit; when you stop to consciously care about what you're going to say then everything falls into place automatically and you've reached fluency. Don't be too eager to rush into a conversation if you're still not sure about the quality of your output, nobody is forcing you to speak. Oh, and have LOTS of FUN! ;) p.s. to all the grammar aficionados: this is just my opinion, so please don't get all mad when I say that grammar might as well be the biggest hindrance to language aquisition ;)

I think you mean: "Do I need to pay attention to grammar as long as they understand what I say?" It depends on your goals. Why are you learning English? If you are looking for informal chat with other English speakers, you only have to worry about basic grammar forms that help you communicate. You can say, "Last week I watch the most scariest movie" and everyone will understand you. However, you can't say, "Last week, the scary movie watched me." So you need to pay attention to SOME grammar. If you want to do something more complex in English beyond chit-chat, then you should pay attention to the grammar when you are speaking.

In that case, yeah, hell, you better pay some attention when you are speaking to some one. Although they can understand you to the most part, if your grammar is not too broken to understand, you better forget about the bs "grammar is nothing, don't give it a damn". That is because, you don't yet know what grammar really is. A quick question: Do you pay much (or even a damn) "attention" to the Chinese grammar when you are speaking Chinese?