What does shay mean when she tells tally to make the worst mistake?

Soon, she comes upon a second chasm, foreshadowed by the line of Shay’s note that reads, “At the second make the worst mistake.” Tally puzzles over what the worst mistake could possibly be. Everything is dangerous out here. She goes to sleep and dreams that she is flying on her own, no hoverboard. That is, until she falls into a deep rift in the cliffs and drowns. She wakes, screaming her defiance, to the impending night and the vast ocean. Tally must find a way across the chasm to continue her journey. She decides to follow the chasm as it dips down on her hoverboard, hoping to find iron deposits in the cliffs to keep her aloft. She goes too far and the hoverboard loses power beneath her. As she was in her dream, Tally is in free-fall. Thankfully, the iron deposits in the water at the foot of the cliffs catch her. This, Tally now realizes, is what Shay meant by “make the worst mistake.”