While one secret about The Walking Dead's zombies wasn't initially revealed to fans, it was planned from the start, as shown by impressive attention to detail in some of its earliest moments.
The Walking Dead focuses on the downfall of society after an outbreak of zombies turns the modern world on its head. Initially, anyone bitten by a 'Walker' quickly turns into a flesh-eating monster, requiring Rick Grimes' band of survivors to destroy their brains to eliminate them as a threat. However, it's later revealed that whatever is causing the zombie outbreak, everyone is already 'infected,' and even a normal death will see them rise again. While this fact wasn't clear from the start, small moments show it was always planned.
In the letters page of issue #29, a fan named Chris Johnston mentions that he had gone back to reread some of the series first issues. Chris asks about the farm family Rick discovers in The Walking Dead’s second issue (drawn by Tony Moore), particularly why they didn't come back to life as zombies. Writer Robert Kirkman points out that if fans pay enough attention, "EVERY SINGLE dead, non-moving body that has appeared in this book since the beginning has had a head injury destroying the brain... this was all planned from the beginning." Indeed, if fans look back to the issue in question, the family have very visible injuries that show why they stayed dead after being gunned down by their father.
Despite being easy to overlook, these details suggest Kirkman's zombie claim is totally accurate, and the creators always knew that every character would become a zombie on death unless they sustained this specific type of injury. What's so fun about this hidden detail is that it has no reason to raise any suspicions - the characters are dead, after all, so injuries make sense - and yet manages to justify the later twist that ever human is already a zombie waiting to happen.
Not only is The Walking Dead a rich story full of symbolism, it's meticulously thought out to the smallest detail. Characters have arcs that make thematic sense to their journey and almost every plot thread pays off in a satisfying way. Of course, like any zombie apocalypse, The Walking Dead needed rules to help tell its engaging story, but it needed to be told in a satisfying and logical way. Slowly communicating how Walkers are revived and what prevents them from rising is just one example for how great the series was at showing and not telling. The Walking Dead's insane attention to detail not only hid a subtle clue, it gave its most devoted fans a reason to go back and see what else they may have missed.