Since large no. of users were reporting GreenPois0n Initialized error in GreenPois0n RC5, Chronic Dev team has now pushed a new fixed version of their tool. Pod2g has confirmed that the GreenPois0n RC5_2 tool fixes the reported bugs and is available for download.
However, they haven’t included Windows and Linux support yet, so be patient. If you want to get untethered iOS 4.2.1 jailbreak with Mac OS, here’s a simple guide that you need to follow.
Download GreenPois0n RC5_2 to fix GreenPois0n Initialized Error
[We've removed the downloadable links from our site due to policy violation issues. Sorry for the inconvenience.]
If you’re facing Cydia related errors, it will be fixed soon.
Update: Download GreenPois0n RC5_2 for Windows