The action of failing instantly at a tried action.
For instance, the act of not putting sage into the email field, and then posting will amount to an instafail, as the poster has failed instantly at saging.
Other instafail incidences can be seen as linking to ebaumsworld, reffering to a picture of a cat with text on it as a "lolcat", or any other commonplace newfaggotry action that takes place on the internet.
Instafailing is not limited to the internet though, any action that can be perceived as failing at the moment the action is attempting can also be seen as an instafail.
Person 1: hey check out this video from ebaumsworld! its halarious!
Person 2: instafail. don't talk to me