when is something an insult?

[Myth]

When it's intended. Stupid people can possibly miss a cunning insult or jab at themselves. If someone smarter explains how they were insulted they have the right to claim it was an insult.

However if a moron takes something as an insult because they are too narrow minded, dumb, uneducated etc. to understand what you meant, it's not your fault that they are a moron and thus should not be held liable for some imagined offense.

Having been the victim of the later I can say it's pretty annoying.