Suppose that in Elbonian word-order is unimportant and "xyzzy" can mean either "stands" or "the zebra" while "zoxas" can mean either "the zebra" or "stands". Sam says "Xyzzy zoxas" in Elbonian, meaning the first word to be a noun and the second a verb. Martha misunderstands his sentence as having the verb first and the noun second. But both versions express the same proposition, that the zebra stands. Thus, Martha misunderstands Sam--she takes him to be uttering a homonymous synonym of what he actually uttered--but she gets right the proposition he asserted.
But what if Sam has no particular intention as to which word is the noun and which one is the verb?