What you may have done is positioned the yarn incorrectly. There are tons of videos out there on magic loop. But not one of them shows you where to place your yarn. I had to figure that out myself. Every time you switch needles, be sure the yarn is BETWEEN the two needles, or coming from the inside of the tube. If your yarn is placed behind the back needle, you'll end up with an unwanted yarn over between the last stitch of the round and the first. I suspect this is what happened.
Welsh corgis are cool.