The 'Yautja Incursions' is a name given to several attacks, deaths and massacres carried out by Yautja Hunter-Killers.
The Yautja are a predatory race, and they take pride and honor in their kills. Occasionally, Yautja will remain on planets for years at a time, picking off soldiers who enter it's territory. Because of this, there have been several reports of deaths caused by the Yautja that have gone unnoticed (or ignored) by the League.
Although several Yautja have been killed by mercenaries for their crimes, none have ever been captured and those that are killed explode with enough force to obliterate a skyscraper.
To this day, no Yautja have been tried for their crimes. Despite protests from League citizens, the League military refuses to see these incursions as an act of war. The incursions stopped after the Yautja-Flood War.