One of the most exciting (and frightening) aspects of building a Windsor-style chair is the ever-present possibility that you will split the seat when you drive the legs home.
I always tell students: Keep hitting the leg’s tenon into its mortise until the very next strike will split the seat. Then stop.
This is not always possible. Today, I split a seat twice. Both splits occurred in mortises for the front legs. That’s typical. And it’s an easier fix than the back legs (more on that sort of repair at the end of the entry).
When you split your seat, immediately remove the tenon from the mortise. The glue can still set up and make things worse. If your seat didn’t completely split into pieces, you are going to have to figure out a way to open the split (gently) so you can get glue in there.
Luckily, you are holding a fantastic lever in your hands: the chair’s leg.