Evasi0n Jailbreak app opens and then closes immediately. Is this happening to you? Some users are reporting that evasi0n jailbreak app crashes when opened and does not jailbreak the device. While there can be many reasons for that, the most common reason could be that the device you are trying to jailbreak may already be jailbroken. If you have tethered jailbreak using redsn0w, then this could happen.
How to check if device is already jailbroken? If you have an app called Cydia on your home screen it means that the device is already jailbroken. To confirm it further, you can connect it to the computer and launch evasi0n app which will show the jailbreak status of connected device and will confirm if the device is jailbroken or not.
Fix Evasi0n Jailbreak App Crash (Exits Immediately)
Download iOS 6.1 IPSW and restore it to your device using iTunes.
After installing iOS, download evasi0n 1.0 for Windows / Mac / Linux.
Now jailbreak your device using evasi0n: evasi0n jailbreak tutorial.
This should fix the issue with evasi0n jailbreak app. The problem, in most cases, appears to be already jailbroken device. Restoring to iOS 6.1 will take care of that and will unjailbreak your device.
If you are still having problems, do let us know in comments.