As a child, I was angry at my older sister and hid her diary in our parents' room as revenge. I expected her to panic while searching for it. Instead, my mother found it and read enough to discover that my sister had been secretly dating someone.
The consequences were huge. My sister lost freedom for months and the entire house became tense. Nobody ever learned how the diary got there. She blamed herself for being careless. I was 11, but old enough to know I had caused it. I still haven't confessed.