Landing pull request 520. Unset the complete function just before calling it to avoid an exception creating a loop. Fixes #5684.

More Details:
 - https://github.com/jquery/jquery/pull/520
 - http://bugs.jquery.com/ticket/5684

src/effects.js

@@ -481,11 +481,11 @@ jQuery.fx.prototype = {
 
 	// Each step of an animation
 	step: function( gotoEnd ) {
-		var t = fxNow || createFxNow(),
+		var p, n, complete,
+			t = fxNow || createFxNow(),
 			done = true,
 			elem = this.elem,
-			options = this.options,
+			options = this.options;
-			p, n;
 
 		if ( gotoEnd || t >= options.duration + this.startTime ) {
 			this.now = this.end;
@@ -525,7 +525,15 @@ jQuery.fx.prototype = {
 				}
 
 				// Execute the complete function
-				options.complete.call( elem );
+				// in the event that the complete function throws an exception
+				// we must ensure it won't be called twice. #5684
+
+				complete = options.complete;
+				if ( complete ) {
+
+					options.complete = false;
+					complete.call( elem );
+				}
 			}
 
 			return false;

test/unit/effects.js

@@ -1205,3 +1205,35 @@ test("callbacks should fire in correct order (#9100)", function() {
 				}
 			});
 });
+
+asyncTest( "callbacks that throw exceptions will be removed (#5684)", function() {
+	expect( 2 );
+
+	var foo = jQuery( "#foo" );
+
+	function testException() {
+	}
+
+	foo.animate({ height: 1 }, 1, function() {
+		throw new testException;
+	});
+
+	// this test thoroughly abuses undocumented methods - please feel free to update
+	// with any changes internally to these functions.
+
+	// make sure that the standard timer loop will NOT run.
+	jQuery.fx.stop();
+
+	setTimeout(function() {
+
+		// the first call to fx.tick should raise the callback exception
+		raises( jQuery.fx.tick, testException, "Exception was thrown" );
+
+		// the second call shouldn't
+		jQuery.fx.tick();
+
+		ok( true, "Test completed without throwing a second exception" );
+
+		start();
+	}, 1);
+});