According to info posted on The Consumerist website, there is, "a McDonald's secret menu item that can only be ordered during the changeover from breakfast to the standard menu. The Mc10:35 is the unholy but oh-so-hot union between the McDouble and Egg McMuffin."
Named "Mc10:35" because that is the only time it can be crafted, the sandwich is NOT a McDonald's offering, but a fastfood mashup of two existing products. It reportedly is sweeping the San Francisco area.
Sounds tasty! Let us know (comment) if you've tried this.
Image: The Consumerist website.