Originally Posted by h6x
which Rom you're trying to flash? make sure that the zip file is right, or is there another zip in the zip file?
I think I have found the solution. I figured there might happen something with the zip files after they are downloaded when my Mac OS X Yosemite opens them (I re-compress the folders usually and transfer the new zip file to an external SD.) So I turned off safe opening on safari preferences, re-downloaded the rom (this time: Pac Man ROM), flashed, and it worked like a charm!
So either you're running Mac or Windows, auto-expanding might cause trouble with the zip files.
I hope others will find this helpful as well.