You didn't ask a question, but I'm going to assume you want to know how to fix it? How far did you go beforeyou discovered you'd joined? Two ways you can do this.
- Tink back stitch by stitch till you reach the point where you joined and undo it then continue knitting flat.
- Take the needles out and rip back to a point whee you joined and then continue knitting flat.
If it's a very detailed/complicated stitch pattern you need to make sure you know what row you are doing when you restart.