There is a hidden feature that allows Pit to haggle with Shopkeepers to lower the prices of items.

In the NES release, this is done by simultaneously pressing A and B on a second controller. In the Famicom release, haggling is instead done using a second controller with a microphone on it, by holding A on the second controller and speaking into the microphone to haggle. In the 3D Classics port for the 3DS, haggling can be done by simultaneously pressing the A button and either Start or Select.

The main catch to haggling is that the prices will only be successfully lowered if Pit's health is one point higher than the first number of the stage he is currently haggling on (i.e. Pit's health must be at 2 arrows when haggling in World 1 to successfully haggle). Otherwise, the Shopkeeper will raise the prices instead.

However if you fail to haggle, the prices will not change until the Shopkeeper's text finishes scrolling, creating a small window of time to buy something at its original price just before they increase.
Contributed by MehDeletingLater
The enemy 'Komayto' is a reference to the Metroids in Metroid, a game that uses the same engine as Kid Icarus, and was released on the same day. As well as having the same appearance, the name comes from Kometo, short for Kometoroido, which means 'Metroid Child' in Japanese.
Contributed by Davsto
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In the instruction manual, the enemy "Siren" is shown naked with breasts, including nipples.
Contributed by pokedude720
The majority of Kid Icarus was developed during three months of intense work, where developers would stay up all night, and sleep in the office. They created make-shift beds using cardboard boxes and curtains, and would endure hours of winter cold in the unheated building.
Contributed by DidYouKnowGaming