Action fail means: When someone's attempt to make a bold move or decisive gesture goes spectacularly wrong in a way that's more embarrassing than inaction would have been. An action fail is the cautionary tale version. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.