Demo: http://jsfiddle.net/simplesmiler/nmsc5cry/4/
Explanation: Inside a component with a two-way synced array prop items, if items is assigned after items.push or any other method, then the assignment gets ignored.

Foreseeing the question "why mutate before assign", there are use cases. For example:

this.items = this.items.splice(start, length); // keep only elements from `start` to `start+length`

EDIT: Actually use case above can be solved without mutation, using slice instead of splice.

Credits to @Kerruba for raising the issue in chat.