Many of you sound like my mother's generation. I'm 73 so you can imagine how ancient you sound.
Why can't you accept that the shoulder slipped?
Maybe she likes to wear strapless bras because the straps cut into her shoulders. By the way, if you looked closely at the pix you would see the folds in the fabric on her right arm which suggests that it was a sleeve that slipped.
I'm writing as someone who doesn't wear a bra most days, unless I'm going out or we are expecting visitors because I find it more comfortable.
Re Corbyn - he is now wearing a grubby anorak, he is wearing a padded coat with a hood - no marks detectable in the pictures - which was appropriate for the weather and his age.