Sweden's dates once ran several days behind today's calendar. Starting in 1700, it planned to catch up by skipping leap days, the extra February 29s, each moving its dates a day forward.
It skipped one. Then a war distracted the government, and the next ones weren't skipped.
Now it matched neither calendar, so Sweden went back to its old one. In leap year 1712, it added back the skipped day as February 30.