Hm yeah, I actually didn't think about that for some reason. But for me, it's so slight that it looks like a mistake, whether it's the bridge or the actual horizon that's slanted. This turned out to be more of a subjective thing than I thought, though. Hmm.
I guess the main thing I still wanted to point out though was that I noticed it without the aid of a ruler. So it stood out as uneven or "wrong", even if it's not. I suppose it matters what you want out of the photo: whether you want it to be technically accurate or visually pleasing or both.