it might be, that the passed function invokes the function again.

e.g.

myDiv.onclick = once ->
  console.log "hello world"
  myDiv.onclick() # recursive loops ftw, all your CPU are belong to us

would run the myDiv.onclick more than once if you place shouldRun = false AFTER executing the function. This is why you should place it first.

if place fn.apply(this, arguments) before shouldRun = false the test will fail.
thus~

once = (fn) ->
  shouldRun = true
  () ->
    if shouldRun
      fn.apply(this, arguments)
      shouldRun = false

any idea?

oops~ it is so cool and tricky