I don't know, in mathematics we say that we have a K-vector space when it really means K-(vector space).

To be fair, "two-week" isn't wrong... but it's in bad company.

Yes. What they were going for was "red head-scarf", but that is also questionable. I've only ever seen "head scarf" unhyphenated.

I can't remember where I came across it (possibly Gowers' Plain Words), but I like the rule that you should hyphenate to avoid ambiguity but not otherwise. E.g. is "a red head scarf" a red scarf for the head or a scarf for red heads? From context, it should be obvious, but hyphenating "head scarf" would make it explicit. (Of course, you could always avoid the problem entirely by using "headscarf" instead.)

There are some exceptions: e.g., I've read you should always write "Douglas-fir" instead of "Douglas fir" because it's not actually a fir.

If you need an hyphen to make clear what a "read head scarf" is, shouldn't you rewrite your sentence?

Well I don't know if this is right, but if I had to guess

red-head scarf might suggest that the flairs at the end of the scarf are red, and that it just a normal scarf https://www.google.ca/search?safe=off&tbm=isch

where as a red head-scarf would be a red... head scarf. https://www.google.ca/search?q=head+scarf&tbm=isch

Dunno. Needs more context frankly.

Yeah, all of those are pretty bad. I would have complained too if I had read this book.

There are one or two errors in usage, which are down to the editor not the writer, but the majority of them are correct (if not always necessary) in British English. His argument was based on the fact that Amazon told him to remove all of them rather than giving the book another proofread and removing the incorrect ones.

Two-week is correct.

Ah, yeah, good point. I didn't read that one.

I read that latter one as specifically meaning a scarf made out of red head