This happened when i was 7 years old.
I lived in suburban midwest US, where all the kids in the neighborhood could walk alone to the park and each other's houses well across town.
My best friend lived on the other side of the park and for some reason I had this idea that it would be fun to write her an anonymous letter giving hints about which friend I was,
and then inviting her to meet me at the park later to see if she had guessed correctly
I decided that the letter had to be typed so the handwriting wouldn't give me away. This was 1997 and computers were so new that microsoft word was entertaining.
I had played pretend secratary enough that I had taught myself to print addresses on an envelope.
I typed up this long letter and gave her 3 clues describing who I was and set a future date for her to meet me.
A week or so goes by and I've pretty much forgotten about the letter when we get a frantic phone call from my friend's mom asking if I had received any suspicious mail.
She was so upset when she saw the letter that she went immediately to the police.
Due to the typed envelope and the advanced grammar (thanks Mom) the police believed there was no way this letter was from a 7 year old but surely a local child molester.
Then undercover agents were sent to stake out the park and wait for his arrival.
Her mom was calling all the parents in town warning them to check the mail and be on the lookout for local predators.
I got grounded *for lying about my involvement at first* for quite awhile and was scared straight after I had to tell my story to the policeman. Whoops.
*Edit: Yes, everyone, I know computers existed well before 1997