diff --git a/test/functions.js b/test/functions.js
index b37fc8976..83c69d730 100644
--- a/test/functions.js
+++ b/test/functions.js
@@ -107,14 +107,13 @@ $(document).ready(function() {
     setTimeout(debouncedIncr, 150);
     _.delay(function(){ ok(counter == 1, "incr was debounced"); start(); }, 220);
   });
-  
+
   test("functions: once", function() {
-    var addBang = function(str) { return str + "!"; };
-    hi = "Hi";
-    hi2 = _.once(addBang, hi);
-    hi3 = _.once(addBang, hi);
-    equals(hi2, "Hi!");
-    equals(hi3, undefined);
+    var num = 0;
+    var increment = _.once(function(){ num++; });
+    increment();
+    increment();
+    equals(num, 1);
   });
 
   test("functions: wrap", function() {
diff --git a/underscore.js b/underscore.js
index cfe7cea44..e214dbd51 100644
--- a/underscore.js
+++ b/underscore.js
@@ -474,17 +474,17 @@
   _.debounce = function(func, wait) {
     return limit(func, wait, true);
   };
-  
-  // Runs a function only if it's never been run before.
+
+  // Returns a function that will be executed at most one time, no matter how
+  // often you call it. Useful for lazy initialization.
   _.once = function(func) {
-    if (_.indexOf(onceRunFunctions, func)) {
-      onceRunFunctions.push(func);
-      return func.apply(func, slice.call(arguments, 1));
-    }
-  }
-  
-  // Internal record of functions that have been run via `_.once`.
-  var onceRunFunctions = [];
+    var ran = false, memo;
+    return function() {
+      if (ran) return memo;
+      ran = true;
+      return memo = func.apply(this, arguments);
+    };
+  };
 
   // Returns the first function passed as an argument to the second,
   // allowing you to adjust arguments, run code before and after, and