Yes rigs can get ewes pregnant. Usually happens when the winter temperature drops below -10C. (We and several other local farms all had unexpected lambs on May 11th one year! lol. When we checked, it had dropped below -10C on Dec 15th the previous year.)
When you ring them, don't remove the elastrators unless you are sure you have both stones. Then check that you have both stones again after you remove the elastrators and before you release the lamb, and if you don't have 2 stones, cut the band off and try again the next day.