Assassinated Archduke Franz Ferdinand, which triggered a chain of events that resulted in World War I. I'd intended to go back to June 28th, 1934 and assassinate Hitler on the Night of Long Knives--thus averting World War II--but was sleepy and mis-keyed the year, ending up in Sarajevo on the same date in 1914. I'd slept through the transit so when the annuciator pinged the arrival, I just got out of the Chronopod and shot the first guy I saw wearing a mustache. I kinda accidentally shot his wife, too, because the wide-beam setting was a smidgen too high.
What a goof, right? Hey, I was tired.
Of course there was an investigation. The Commission didn't pull my time cert because history showed that a guy named Gavrilo was going to shoot the Archduke anyway, but you can bet it delayed my next promotion by about two decades and my Trans-Temporal Reviewer time was cut to nothing for 60 days. (Not that there's anything good to watch, even with over 14 million alternate timelines to choose from.)
It's okay for me to tell you this because next week, [redacted] will [elided] [redacted] and the entire Earth will [redacted]; nobody will take you seriously if you repeat the story anyway.