You need to be able to solve things ahead of time if you're paying attention. A mystery that cannot be realistically solved without extreme guesswork before the answer is given to you, is a poor mystery because mystery is fundamentally an interactive genre, and unsolvable mysteries disincentivise that interactivity.

Also, if a story becomes worse with knowledge of plot twists and later story info, THE STORY WAS ALWAYS BAD EVEN THE FIRST TIME. Spoilers should not ruin stories, and if they do ruin stories, the story always sucked.