Terrific explanation of the inner-workings of the #CommonLisp condition system (a powerful superset of #exception handling that can do way more cool stuff, like restarts): https://www.youtube.com/watch?v=pkqQq2Hwt5o

Basically, the condition system itself is built directly in Common Lisp using facilities in the language that are available to the user: dynamic variables, closures, and non-local flow-control (tagbody/go and block/return-from). Nice syntax/structure provided by macros, of course. Fucking brilliant.