Dev Team has released GreenPois0n RC5_2 for Mac. It contains fixes of various issues occurring in GreenPois0n RC5. GreenPois0n RC5_2 can also be used as untethered jailbreak for iPhone 4, iPhone 3GS, iPod touch 4G, iPod touch3G and iPad running on iOS 4.2.1 without SHSH blobs of iOS4.2b3.
So instead of using GreenPois0n RC5 which was first released earlier. This news was confirmed from @Pod2g and @iH8Sn0w twitter’s account. So try this tool instead of using the first release.
Download GreenPois0n RC5_2 for Mac OS X.