By way of Joe Marshall in comp.lang.lisp:

Here's an anecdote I heard once about Minsky.  He was showing a student how to
use ITS to write a program.  ITS was an unusual operating system in that the
`shell' was the DDT debugger.  You ran programs by loading them into memory and
jumping to the entry point.  But you can also just start writing assembly code
directly into memory from the DDT prompt.  Minsky started with the null
program.  Obviously, it needs an entry point, so he defined a label for that.
He then told the debugger to jump to that label.  This immediately raised an
error of there being no code at the jump target.  So he wrote a few lines of
code and restarted the jump instruction.  This time it succeeded and the first
few instructions were executed.  When the debugger again halted, he looked at
the register contents and wrote a few more lines.  Again proceeding from where
he left off he watched the program run the few more instructions.  He developed
the entire program by `debugging' the null program.