There are a few, but from my experience (working with high-level English learners, helping them to go from “okay…” to “amazing!”) these are some of the biggest:

1. All study all the time, with no using (which results in slow, academic English).
2. All using all the time, with no focused study (which results in fast, but very messy, chaotic and disorganised English which is hard for people to understand).
3. Using the wrong kind of materials for your goals (e.g. studying newspapers or news broadcasts and expecting to get good at conversation… which is very different).
4. Trying to improve accuracy and naturalness by studying more grammar (but that’s not how natives speak — “could you aid me in this task” is grammatical, but very unnatural… “Thanks very much” is ungrammatical, but very natural).
5. Thinking memorising more words will make you fluent (it won’t).
6. Living in an English speaking country and expecting that to be enough to improve (it won’t).
7. Doing nothing (you’d be amazed at how many people I talk to who complain that they don’t improve, but are actually doing nothing to make it happen).
8. Doing the same things, and expecting different results (if you’re not progressing in English… you need to change the things you’re doing).

Now, the fast way to improve? Use a method which works, materials that are going to give you natural samples of English you’ll actually use, and change the way you think about English (because it’s probably wrong). I’d also add to this, work with a coach who can help guide you and show you how to fix your personal problems quickly and easily. On the first few points (method, materials, and thinking) I’ve got a free training here that will teach you the nuts-and-bolts of improving as an intermediate or over English learner — it’s here. Hope that helps. Best, Julian Northbrook