John Berryman's Dream Song 29 is often remembered for its last line, but I prefer its first. I think there's something special going on with its syntax: the reader has to subconsciously decide whether the PP "on Henry's heart" is modifying the noun "thing" or the VP "sat down", and there's a sort of pleasing oddity to that.
It's most natural to interpret "on Henry's heart" as modifying "thing" because it comes right after. It's adjacent. There's more cognitive effort in processing a modifier if it's in some way displaced from the phrase it modifies, and why expend the effort if you don't have to? So the most immediate and natural interpretation of the syntax is this:
But if we reconstruct the sentence as "A thing on Henry's heart sat down once", we see semantic awkwardness: an object already on Henry's heart would not need to sit down because it's already sitting down (on his heart). You wouldn't say "a man on a bench sat down" or "the woman on the boulder climbed up", and "a thing on Henry's heart sat down" has the same problem. It's semantically more natural, though syntactically more awkward, to read the line like this:
which reconstructs as "A thing sat down on Henry's heart once." There, no redundancy!
Of course, this isn't really the whole sentence. If we consider a "sentence" as "a length of text ended with a period" the whole sentence is
There sat down, once, a thing on Henry's heart
só heavy, if he had a hundred years
& more, & weeping, sleepless, in all them time
Henry could not make good.
where lines 2-4 are just a long PP modifying "thing". I feel like what these lines do regarding the syntactic/semantic tension is disincline the reader to immediately notice any awkwardness, because the train of language has kept on moving. If the poem's first sentence was just "There sat down once a thing on Henry's heart," full stop, end of sentence, the tension would be more jarring.
I feel like there's something magical about this little flourish, this brief micro-tension between syntax and semantics; I find this sentence more interesting than the more overt syntactic oddities you sometimes find in Berryman's poetry, including the rest of Dream Song 29.