I agree with Steve that the biggest mistake in language learning is the "illusion that we need to master anything." Yet I think that we can "master" certain aspects of a language on micro levels, i.e. numbers, days of the week, greetings, etc. Yes, we do not "need" to master anything, but we can master parts of the whole; the rest being new and wonderful.
What's more, I think that language learning is an ever evolving lifetime study. When we pressure ourselves to "master" that which naturally evolves over time and with constant effort, we create an environment rife with frustration and anxiety, impeding the learning process.
Among the common mistakes in language learning is relying 'too long' on one's native tongue. (I'm guilty!)
It is my understanding that learning a language requires taking a leap into that language. At some point in our familiarity with the target language, we need to let go of our native language, stop translating in our heads. (Here, I am not referring to the work of translators, or translations.) When we have built up enough "identifying markers" through intense listening and reading, it may be necessary to begin a different kind of mental training: developing and deepening our knowledge of the target language via the target language, learning how to meld one's mind and spirit to the body of the target language.