I heard about this from a cashier at one of the local McDonald's. He said it's getting kind of a cult following in San Francisco.
1. Go to McD's right when they are transitioning from breakfast to lunch.
2. Order one of the remaining Egg McMuffins from the breakfast menu and also order a McDouble since the lunch menu is now open.
3. Take the egg and Canadian bacon from the Egg McMuffin and put it on the McDouble.
The guy at the register said people call it a Mc10:35 because that's pretty much the only time you can pull this off. Via