Final answer:
The memory sin of the national memory champion who uses post-its is termed 'transience,' which is the natural fading of memories over time.
Step-by-step explanation:
The memory sin that the national memory champion might be demonstrating by living by post-its in her personal life is known as transience. Transience refers to the forgetting error where memories can fade over time. For example, when someone reads a book like To Kill a Mockingbird in high school but years later can only remember the names of the characters and not the plot, this is due to storage decay, where unused information tends to fade with time. Similarly, relying on sticky notes indicates that the memory champion is aware that some information may not be recalled easily and hence uses external reminders to prevent forgetting important details, which aligns with the concept of transience.