In 2007 Switzerland accidentally invaded Liechtenstein, realized their mistake and apologized! A routine training exercise conducted by Swiss soldiers in the middle of the night got them lost.
They ended up in the neighboring country of Liechtenstein. All 170 infantry soldiers crossed the country’s border at an unmarked area and made it 1.5 kilometers inside the country. Luckily, they soon realized their mistake and turned back.
Though the soldiers had assault rifles, it was said that they were not carrying any ammo and both the Swiss and Liechtenstein governments acknowledged and played down the incident saying that no harm was done. Lucky for those troops, too!