I just heard about this type of sentence, called a depth charge. Parsed carefully, this sentence should mean "all head injuries should be ignored, even if they seem trivial enough to treat."

I couldn't believe that at first. But consider these sentences (regardless of whether you agree):

"No dog is too old to be eaten."

"No gun is too dangerous to be sold."

They imply:

"All dogs should be allowed to be eaten." (Regardless of age)*

"Dogs should (or can) be eaten regardless of how old they are."

"All guns should be allowed to be sold." (Regardless of dangerousness)*

"Guns should (or can) be sold regardless of how dangerous they are"

Therefore the top sentence means "All head injuries should be allowed to be ignored!" (Regardless of triviality)

Here's an article about why we may (mis)interpret these sentences this way.

Basically what I think is going on is that a complex sentence that is grammatical will be assumed to have the most logical meaning. But I don't really know. More people have read the article than I have!

*edit: see this comment for why I changed these.