IfDo you mean Twilight having two strands of mane around her neck? I don't think that's necessarily a mistake. I've seen some artists depicting her mane like this. For example:
- [hioshiru](https://derpibooru.org/images/2134715)
- [magnaluna](https://derpibooru.org/images/2207303)
I think her mane *can* flow like that if you imagine Twililight splitting her mane into two (it's physically possible).