I always figured it would be incorrect to use "however" in the middle of a sentence, since that's what "but" is for. However, I've seen it in the middle of sentences before, with a comma. Such as, "I was walking and was aimed towards going to get the mail, however I ended up at a strange garden instead". I've actually seen that, in books and in news, etc. I've always wondered if it was correct. However, I stick to using "however" in the beginning of sentences.