United StatesI took the time to find and show folks things I found interesting in NES games. 4 posts in 4 NES games. It seems the posts enraged folks and all but this one have been deleted without so much as a message. Do not fret, this "mf" will not be posting anything anymore. It is clear this human is not wanted on this place. Thanks.
If you get the hammer from the hammer bros and smash the rock you can enter the the top right part of the map. There you will see III the roman numeral 3. This is likely celebrating Mario 3!
So you might know about the minus world "glitch" where you go to a never ending water area that looks like 2-2. But, what you might not know is there is more to the minus world on the Famicom Disk System version of the game. In fact, in that version you can progress beyond -1 and all the way to -3. Its a trip and you should check it out!
Sources:
VGMaps Super Mario Bros World -1 NES map - NES/Famicom Cartridge Version
VGMaps Super Mario Bros World -1 NES map - Famicom Disk Version
VGMaps Super Mario Bros World -2 NES map - Famicom Disk Version
VGMaps Super Mario Bros World -3 NES map - Famicom Disk Version
This secret might be contentious but there is a Swastika (also called 卍 or Manji) that is "hidden" in Zelda. If you take a look at the shape of the First Quest Level 3, you can see for your own!
Source: VGMaps The Legend of Zelda First Quest Level 3 NES map