lodash.src.js (as of commit 5613f60) has this at line 7437 (the beginning steps of _.debounce):

  wait = wait < 0 ? 0 : wait;

If you call debounce without a wait argument, i.e. _.debounce(f), the variable wait remains undefined. A bit later (starting at line 7457) there is this:

  function delayed() {
    var remaining = wait - (now() - stamp);
    if (remaining <= 0 || remaining > wait) {
      // cleanup and call debounced function 
    } else {
      timeoutId = setTimeout(delayed, remaining);
    }
  } 

Now, because wait is undefined, the value of remaining becomes NaN, and the test right below it is always false. Thus the delayed function is set as a timeout again, which is does the same thing again, etc. Basically it loops indefinitely.

I propose changing line 7457 to

  wait = wait >= 0 ? +wait : 0;

which forces wait to be zero if it is undefined.