At the title screen, after "press start" has started flashing, press A, A, B, B, Right, Down 7 times, Left and Start. If done right, you'll be at a special debug menu where you can select ...

You'll have the option to start a normal or "NoHit" (invincible) game. Pick either one, and some numbers will appear on screen. These function both as a sound test and as a level select.

All three of these games are incredibly rare, but Hot Slots is the most uncommon and thus the most valuable. One of the rarest standalone titles on the NES is The Flintstones: The Surprise at ...