I'd like to see what David can tell us, but the only sentence that sounds correct to me is (4) (meaning: I was afraid of breaking the expensive plate).
1) The accident I was worried about happening finally happened.
2) The expensive plate I was worried about breaking broke.
3) The accident I was afraid of happening finally happened.
4) The expensive plate I was afraid of breaking broke.
In '2' was I worried that the plate might break or that I might break it?
In '4' was I afraid that the plate might break or that I might break it?
Hello, Navi and Gustavo—This is a very interesting question. As usual, Navi is pressing at the boundaries of the syntax of the English language. I think that there is very, very little published about this issue in the literature.
My gut reaction is the same as Gustavo's: the only sentence that sounds correct to me is (4), with the interpretation that it was the speaker who was the potential plate-breaker ("breaking" as transitive, not intransitive/ergative).
But are the other sentences ungrammatical? What Navi is trying to do here, whether or not he realizes it, is to take an ACC-ing construction and extract the subject of that construction via wh-movement in a relative clause.
In Stephen Abney's renowned dissertation, "The English Noun Phrase in Its Sentential Aspect" (1987), he discusses the ACC-ing construction a little bit (see p. 169), citing some of its properties with reference to a 1983 article.
In that 1983 article, Eric Reuland (the author) maintains, according to Abney, that "wh-movement from subject" is permitted in the ACC-ing construction. Abney gives the example "Who did you approve of t studying linguistics?"
To me, that sentence just as grammatically questionable as Navi's sentences (1), (2), and (3). It is understandable, but sounds off, and as though it should really mean "Whom did you approve of while studying linguistics?"
Because I have only been able to find one source saying that subject extraction from an ACC-ing construction is permitted in constructions with wh-movement, and because I think it sounds bad, I'm content to call it questionable.
Interestingly, if we allow Navi's maneuver here, both (2) and (4) will become ambiguous, having both the readings that Navi has mentioned. Is the plate potentially breaking of itself, or is the speaker potentially breaking it?