Forbidden Taboo

''What did I tell you about reading from that forbidden book? Now you'll be cursed for life!''

The thing someone (likely a member of the Main Cast) was warned against but still associates with, resulting in inevitable badness. The Forbidden Taboo comes in many forms, from food to places to weapons to gear. Whatever form it takes, it shouldn't be messed around with. But since Plot Forced UnTrash, the Main Cast must go against their warning and mess around with it. The Forbidden Taboo might summon a baddie, rip a portal through spacetime and whisk everyone away to some pocket dimension of evil, curse them, or cause a Nuclear Explosion.