If you cross the border of the restricted area surrounding Area 51 FOR ANY REASON, the security guards, known as Cammo Dudes, will detain you. You will be pulled out of your vehicle, and held at gunpoint, face down on the ground. The cammos will call the Lincoln County Sheriff.
When the sheriff shows up (usually about an hour later) he will arrest you and you may get to see the inside of the Lincoln County jail, while the sheriff investigates your story.
If he concludes that you accidentally crossed the line, you will be released after receiving a $650 citation for trespassing and a notice by the "Installation Commander" of an unspecified "Military Installation" that you are not welcome there. In more severe cases the fine may be up to $5000 and possibly up to 6 months in jail.