Sit back and relax a while, and let me tell you a story...a sad story of OptiFine and how it loved sending me for unexpected visits to the desktop.
I had ugly nasty horrible frustrating insta-crashes for a while. These are the steps I attempted to fix it:
Clean install of Java 7.9 (I'm on PC). A very serious pain to get a proper clean install of Java 7.9 for 64-bit Windows 7. No fewer than 3 applications promising to fully and properly uninstall Java and all of its grubby little tendrils were tried until I finally found some obscure little app that got the job done. It was during this time that I learned Mozilla had abandoned support for 64-bit Firefox, meaning that when I updated Firefox to the latest version it actually reverted to a 32-bit application and the detection applet on the Sun/Oracle/whatever site where you download Java kept giving me 32-bit Java as a result. Eventually, I was the proud owner of a clean 64-bit Java 7.9 install and it did absolutely nothing for my problem.
After poking and prodding google for a while, I learned that perhaps I had gotten a wonky version of lwjgl (Light Weight Java Games Library, an integral part of Minecraft) so I found, downloaded, and installed the most recent version. Still no luck.
Ultimately I found myself on the OptiFine thread at the official MC forums where I was sadly not surprised to find that a dozen or more solutions had been presented and none of them worked.
And then, off in some far flung corner of the interwebs I found a glimmering little tidbit, the foretold solution and the key to my victory over the bitchy little nemesis that is OptiFine:
Launch FTB
Go to the Options menu from the main Minecraft screen.
Go to Video Settings
Find the option called "Chunk Loading"
Set it to anything that is NOT Multi-Core.
Also, if you have an nVidia GPU, go into your nVidia control panel and disable threaded optimization.
Good luck!