Budd's Boots and why she couldn't kill him
I pointed this out years ago but figured I jump my theory to the top again.
Before The Bride is put into the coffin she's wearing dark brown boots. However in the coffin she's wearing a completely different pair (you can easily tell this) that happened to have a razor in them. It's very unlikely Tarantino made a mistake that big.
My theory is Budd clearly put the razor in those giving her the possibility of escape. I'd also point out that Budd did it as an act of revenge the same as The Bride tried to kill him.
It's the reason The Bride couldn't kill him. Budd wanted revenge for his brother and The Bride wanted revenge for her family. They cancel each other out.