Okay, this is what I call the amazing increase phenomenon. I used to get this all the time. When your using garter stitch and shoving of the old loop sometime it won't make it all the way off, and the yarn either splits so some, but not all is on your needle or just flies on the other needle and then you make a mistake and knit it. What I have found helps with this problem is to count you stitches that you have cast on. That way when your not sure if you should knit a stitch you can see if it was there before. If it was not there before the just gently lift it off the needle. Using bulky yarn makes t even more easy to catch. Happy knitting! Good luck!