Fear is a funny thing. It can take many shapes and forms and propel us to do things we'd never imagine ourselves capable of. But sometimes, it is the absence of fear that we should be most afraid of.
This was the dilemma of Mary Queen of Scots. While in prison, she sent encoded messages to a group of people working to free her detailing plans and revealing ill feelings toward her sister, Queen Elizabeth. She wasn't afraid of her secrets being discovered, for she trusted the man smuggling the encoded letters in and out of the prison and she believed her cipher could only be understood by those receiving the letters. She was wrong.
This is what was really happening: the man sneaking the letters in and out of her cell was bringing them to a team of cryptanalysts working for Queen Elizabeth, who would crack the cipher, decode the message, then reseal the original letter and send it on its way. Neither Mary nor her allies knew any of this was taking place; they didn't even think it was a possibility someone could figure out their cipher. That's the problem with cryptography-- when you think you have an unbreakable code, you aren't afraid of anything.
Eventually, Mary sealed her fate by sending a letter giving her permission to carry out a plan that acted against Elizabeth and her party. The cryptanalysts decoded it and finally had evidence that Mary was plotting against the throne and therefore could be given the death penalty. This must've come as a shock to Mary, who thought her messages were unbreakable.
When cryptographers believe they have an indecipherable text, they think they have nothing to fear and become comfortable sending and receiving messages at their leisure. This comfort zone is dangerous, and can lead to endings such as that of Mary Queen of Scots. So, when encoding messages, always be extra cautious and never get too comfortable with your ciphers. You could end up headless.