The misogyny in The Fashion in Shrouds. As a teen I adored Allingham's works, she was my favourite of the Golden Age Queens, but reading this book saddened me with its repeated casual misogyny, and while I generally try not to project current mores retrospectively, I was horrified by this:
"“Oh,” said Mr. Campion furiously, “this is damned silly introspective rot. What you need, my girl, is a good cry or a nice rape—either, I should think.”(e.a)

That's an extreme example, but the utter subjugation of Campion's sister to this sort of Neanderthal chauvinism was also depressing, especially given the way Allingham wrote about it as as though it were a good thing:
I would make all decisions which were not directly in your province, although on the other hand I would like to feel that I might discuss everything with you if I wanted to; but only because I wanted to, mind you; not as your right. And until I died you would be the only woman. You would be my care, my mate as in plumber, my possession if you like."

I'm going to carry on with my re-read, but this one in particular was full of constant reminders that women really ARE inferior, in abilities and virtues, apparently, and it took the gloss off my rosy recollections. Especially since I love the Albert and Amanda pairing.