5. Gus Dies
Show: Breaking Bad (AMC)
Not only did they spoil the most important moment to date in an all-time great series for you, but they did it with references to "Two-Face" and The Terminator that they actually thought were funny. Sure, Gus's (Giancarlo Esposito) death was inevitable. To quote Sepinwall once again: "If Walt's going to become Scarface, then at a certain point, Scarface can't have a boss anymore."
The joy of watching Breaking Bad doesn't come from unpredictable plot turns, but from the skillful execution of expected results. If a spoiler merely let us know that Gus died, we could live with that. Spoiling the apparatus that gets us there is an unforgivable TV sin. Nonetheless, being spoiled on Fring's last moments doesn't change the force of the final image of Gus. We suspect that every viewer of Breaking Bad will have the image of the chicken king straightening his tie burned into the back of our minds forever.