What are two incidents from The Story of My Life that show Helen was mischievous?

Even though young Helen Keller was blind and deaf, she still managed to cause a lot of trouble. In her book, Keller describes how as a little girl she was often accompanied by the cook's daughter. Both girls loved mischief. They got into all kinds of trouble together.

We were busy cutting out paper dolls; but we soon wearied of this amusement, and after cutting up our shoestrings and clipping all the leaves off the honeysuckle that were within reach, I turned my attention to Martha's corkscrews (Chapter 2).

Since Helen Keller lost her sense of sight and hearing to an a toddler, she wasn’t really able to communicate with the adults around her as effectively as they would have liked. She used her own special language of signs that Martha Washington understood. Her parents could communicate with her this way, too, but they still found discipline difficult.
The incidents little Helen was involved in were not all harmless.

One morning I locked my mother up in the pantry, where she was obliged to remain three hours, as the servants were in a detached part of the house. She kept pounding on the door, while I sat outside on the porch steps and laughed with glee as I felt the jar of the pounding (Chapter 2).

After this incident and a few others, including one where an impatient Helen caught herself on fire and caused burns to her hands and hair, her parents decided she needed a teacher. When you cannot give a child proper guidance, the child just runs wild. This is especially problematic when that child is as creative, intelligent, and mischievous as little Helen was.
Helen's parents hired Anne Sullivan as her teacher because they were not close to a school for the blind. Sullivan was able to teach Helen proper sign language even though she was deaf and blind, and soon Helen could communicate much more completely.