Another syntax-y issue. I'll call it the dangling 'which', since I'm not sure of the terminology.
e.g. Stan and Fauntleroy are making cakes. Stan makes a superb, exemplary cake, whereas Fauntleroy becomes distracted, leaves something to overheat, goes off somewhere and returns to find half his ingredients aflame. No one is pleased.
You want to convey this displeasure to Fauntleroy, thusly: "You were meant to be baking... something which Stan accomplished, but you not only failed *spectacularly* at, but topped off by setting the cheese on fire."
I see a lot of this instead: "You were meant to be baking... Stan accomplished this, which you not only failed *spectacularly* at baking but set the cheese on fire too."
It works if you substitute 'whereas' for 'which', but it's like the writer/speaker lost track of what their 'which' referred to (i.e. baking), spiralled off into a new thought, and restated a whole unnecessary clump of sentence. It reads as if it's switched tracks suddenly in the middle, and if it's a very long sentence, I lose the thread.