Hmm. What a great question.

    while(1) { fork(); } 

Must have meant something to me, because it was in my Usenet sig. And then:

    exit(main(kfp->kargc, argv, environ));

from FreeBSD crt0; kind of a big moment for me to realize there was more C code underneath main().

Then:

    statements: /*E*/ | statement statements

The moment Yacc clicked for me; that line was profound but a more accurate depiction of the moment was realizing that all you were doing with the parser generator was building a tree, and that's what the side effects of each production were for: returning and plugging in tree nodes.

If there was a single line I could come up with for "double the size of the hash table when it fills up", that'd deserve a slot here.

Then, of course (we get to cheat because assembly):

    jmp .call
    .pop: pop esi
    ...
    .call: call .pop

The "delta offset" trick, which I believe came from x86 virus culture but was universal in 90s overflow exploits as a way to get position-independent references to strings and other memory things.

For sure:

    void walk(node *n) { if(!n) return; walk(n->left); visit(n); walk(n->right); }

I should figure out a way to fit a write-select(2) into a line because that's another "oh shit" moment for me.

Similarly: when Vern Paxson explained to me that he'd implemented TCP reassembly by simply opening up a file and using "seek" to place incoming segments in the right position. I could probably golf that down to a line! But it would be an annoying line.