Sam and Dean's Initials
In 12.18 The Memory Remains, Sam and Dean sit in the library wondering what their legacy will be a hundred years from now. Sam suggests the people they saved will be their legacy, because they left the world better than they found it. Dean wonders about the Bunker, if other hunters will move in. Dean gets out his knife and he and Sam carve their initials into the library table as they had done in the Impala. Later in 14.18 Absence, we see that Mary Winchester's initials have been carved alongside Sam and Dean's initials.