Add comment re: undefined issue

lib/mixins/property-effects.js

@@ -966,7 +966,11 @@ function parseArg(rawArg) {
 
 function getArgValue(data, props, path) {
   let value = get(data, path);
-  // when data is not stored e.g. `splices`, get the changed value
+  // when data is not stored e.g. `splices`, get the value from changedProps
+  // TODO(kschaaf): Note, this can cause a rare issue where the wildcard
+  // info.value could pull a stale value out of changedProps during a reentrant
+  // change that sets the value back to undefined.
+  // https://github.com/Polymer/polymer/issues/5479
   if (value === undefined) {
     value = props[path];
   }