King Cobra in Snake Boxer 5
While on the title screen of Snake Boxer 5, press (on the arrow keys) Up, Up, Down, Up, S to play as King Cobra.
Inside the photo booth, make Strong Bad walk off the far right side. Do the same thing three times, on the third try you should be upside down walking on the bottom of the booth. Walk over to the far right, and stay even with the door. Make sure you're as far back as you can get. You will here a confirmation sound, and Strong Bad will automatically be wearing the bonus shirt. When you take the shirt off It will not return to the rack ,so you will need to do this every time you want to wear the it.