A relative clause is a ‘fill-in-the-blank’ construction: the relativizer ‘points’ not only backward towards its referent but also forward toward a blank space or ‘gap’, a missing entity, in the clause which it introduces.
Ignore the when clauses, which are not essential to the structure. In your second example the relative clause is missing an object for the verb get—I’ve bracketed the clause and marked the gap with __:
maddening snippets of melody which [you can never get __ out of your head].
In your third example what’s missing is the subject of the passive construction be gotten:
... maddening snippets of melody which [ __ cannot be gotten out of your head].
But in your first example there is no gap:
... food which [you say “Now this is real food!”]
You say “Now this is real food!” is a complete sentence, with nothing missing—which means there is nothing for which to point forward toward.
“But” (I hear you say) “Why doesn't it sound wrong when the when clause is there?”
Good question. It's because of the it in the when clause. ...
The rules around what a relativizer like which can stand for get very tricky when the relative clause is long and complicated. In some syntactical situations the rules don’t allow your relativizer to point to the piece you want it to point to.† In speech it happens fairly often that we start a construction with only a vague idea about how it’s going to come together. We don’t realize until we get to the point where we intended to put the gap that a gap isn’t allowed there. When that happens we generally fall back on a syntactic device which linguists call the ‘resumptive pronoun’: rather than start over at the beginning of the clause we just fill the gap up with a personal pronoun having the same referent as the relativizer, like this:
This is the girl that whenever it rains she cries.
In speech it's perfectly ordinary and acceptable. In fact, it’s an ancient device, which was actually ‘standard’ in Old English, around 1000 years ago; it’s ‘ungrammatical’ only in formal writing, where a competent writer is expected to edit his copy to comply with the standard rules.
Now: your example doesn’t have an honest-to-God resumptive pronoun. But it does have a personal pronoun, it, which has the same referent as which and which sits in just the sort of place where you feel like a gap might be prohibited and a resumptive pronoun would be called for. In effect, that it fools the ear (yours and your hearer’s) into thinking that the missing gap has been filled up acceptably; so you (and your hearer) just charge ahead to the end of the sentence and don’t worry about it any more.
It's wrong; but unless you put it in an English LitCrit article probably nobody’s going to notice.
† If you find this sort of thing exciting you can read about it in the Wikipedia article on Wh-movement, especially the really action-packed section on Extraction islands.
