Some blocks / views attach CSS/JS conditionally, i.e. only if the viewed content requires it. For example in the commerce cart view, the CSS file
commerce_cart.theme.css is only loaded when the cart actually has any content (see commerce_cart_preprocess_views_view).
The problem is when the CSS/JS was not attached while a page was saved to the cache. On a subsequent request a personalized block / view may have other content requiring the style/scripts. However these assets will not be available on the cached page and therefore the personalized fragment might not look good / work properly.