It's not something that's really featured in my life recently, but I felt the need to vent my annoyance with books that tell you how to pronounce Japanese. Specifically, the Japanese r-sound, which I'm reliably told is sort of half-way between a 'r' and a 'l'.
That's just silly, isn't it? There isn't some kind of sliding scale with 'r' at one end and 'l' at the other. It's either one or the other. After some experimentation, involving saying "ra la la ra" to myself repeatedly, I can say with some confidence that you touch the roof of your mouth with your tongue to say 'la', and don't to say 'ra'.
In my experience, Japanese people either say 'r' or 'l', depending what they feel like at any given moment, and then insist that they make the same sound every time. Or that's how it seems to my English-speaking ears, anyway. I just say 'l', seeing as I always have a problem with that other sound. And saying 'w' instead of 'r' in Japanese is at least 50% more likely to lead to saying an actual different word, probably. I haven't checked, but it's best to avoid it anyway.