Flashing 2.3.3 'Leaky' onto a Samsung Epic 4G
Anything can go wrong, including issues that permanently brick the device.

That said, Galaxy S devices are compatible with a tool called Odin that can fix all but the worst problems.  Example process.

I personally flash pretty much anything I feel like onto my Vibrant, and Odin has saved me many times.  I wouldn't hesitate to flash something I wanted to try out, but you may wish to be more cautious.