Last month a small European country was accidentally invaded by another country. What two countries were involved?

Are you referring to the embarassing incident when Swiss soldiers got lost at night and marched into neighboring Liechtenstein?.

Well done sr...yes apparently last month 170 Swiss troops got lost while on an exercise and crossed the border into Liechtenstein.

Here's a link to The Guardian

There is a good story about Liechtenstein's army, told by Alan Whicker, a veteran BBC reporter who had to research the principality many years ago. He discovered that the last time Liechtenstein sent her army to war not only did she suffer no casualties but she ended up with more of her soldiers coming back than she'd sent out !She gained or regained an extra ten or so. Perhaps the extra ones were on holiday in the place invaded at the time, so never reported to base, but hitched a lift back home afterwards?

[Her army was disbanded in 1868]