I know exactly what you mean, I saw a girl yesterday with an umbrella and flip flops (or to be more precise Fit Flops, which is probably even more of a crime). It was raining pretty heavily, her feet were very wet, she appeared quite vexed. I didn't have a moment of pity for her, for surely when she was slinging her umbrella in with her oyster and lunch money it might have occured that a closed shoe was the order of the day.
I shouldn't be overly censorious though, as I am not above making a mistake of being over dressed on a warm day. But I never mix and match, thinking erroneously that my top half is in Barbuda while the bottom half is in Vladivostok. She will suffer, and that will be a lesson learned.