iOS 6.1.3 released publicly and the death of the evasi0n jailbreak
Now that iOS 6.1.3 has been released to the public, the days of the evasi0n jailbreak are over. After 6 weeks and 2 successive incremental updates, Apple has finally patched the exploits used in the evasi0n jailbreak. For those with iPhone4 or lower with your SHSH blobs saved, you have nothing to worry about. However for users with an A5 or higher processor, there is no way to downgrade back to 6.1.2 now that 6.1.3 is out and the signing window has closed.
Because of this, it is extremely important that you are cautious if you wish to remain jailbroken. If anything corrupts your firmware, you will be forced to restore to 6.1.3 and be unable to jailbreak until a new exploit is found which likely would not be used until at least iOS 7. Avoid installing tweaks/themes/apps that you are not sure are compatible with your device/firmware.
Luckily, there is a pretty solid safeguard available to help protect against this. If you have MobileSubstrate installed, you will be able to reboot your device into safe-mode by holding the power up button while rebooting. This will allow you to remove the offending tweak from Cydia and reboot to your normal springboard. MobileSubstrate is free and comes with SBSettings. This method will not work for every problem though, you can still delete or alter the wrong file that will cause your device to not even be able to boot into safe-mode and you will then be stuck with restoring 6.1.3
Hope this helps!