Like someone said, every time I write Forth or Lisp :) So I took the next logical step and implemented Forth in Lisp (https://github.com/codr4life/foonline). Why? Because it gives me more leverage, it's like having one more dimension on top of macros. The other reason is that it gives me facts to go with opinions on language design, allows me to see the compromises others made in a new light.