Once upon a time there was a bad emperor named Caracalla. One day, a prophet started ranting that his random assistant would be emperor. The assistant, Macrinus, heard about this, and realized that if Caracalla heard, he would likely kill Macrinus just to be safe. So Macrinus had Caracalla assassinated. And after a bit of palace intrigue, managed to get himself declared emperor.
The lesson: don’t be unreasonable about stupid prophecies, and then people won’t kill you to prevent your unreasonable responses.