You'll get that error for a few possible reasons: 

- You don't yet have an official Minecraft license, but are using something like a Team Extreme Client, which avoids official Minecraft authentication. Our server uses official Mojang Authentication, so attempting to join will throw that error. In other words, if you downloaded your Minecraft account somewhere other than then it may not be authenticated. 

- Your session has been invalidated because you or someone else logged into your Minecraft account.

- Your Minecraft window has been open too long

- Problems connecting to the server -- the internet drops out when authenticating to Mojang.

Unless you're using a problematic cilent, usually the best fix is to exit then relaunch Minecraft.