Refusing to wear a slip, which would almost certainly improve how the dress looked, because you associate it with women over 50....... Probably not the silliest thing I'll see on S&B today but possibly the silliest thing I'll see before 9.30am.
That said - and admittedly I don't wear dresses very often - I wouldn't spend £165 on a cotton dress if it was see through. I'd want either a sturdier fabric or a (cotton) lining.