Probably means the image files you're trying to use are in some way flawed. Either they're corrupted anyway or were made with a software that makes images totally incompatible with standard images...
A Solution to Codec Overrun
Greetings all. A solution to the codec overrun problem is to boot from Linux (I used Mepis 7.0 and the Konqueror Browser and booted on the PeeSea). I then downloaded the same .dmgs that wouldn't load and they worked fine! I had previously downloaded the same files with different Browsers on the PeeSea but had mounting problems on the Mac.
I don't think the problem is the Mac itself but the way in which the data it downloaded/assembled on the Hard drive. I have not been using the Mac for downloading.
I know this is a limited solution (sigh). Here is something that might help you.
If you can boot a Windows session on your Mac you should be able to boot Linux. 'Live CD's' with bootable Linux seem to be popular in my neck of the woods (Australia) in PC magazines. You put them in your CD drive and choose to boot from it. I am vague on the booting on Mac through Windows because I don't run Windows on my Mac (too old, alas). If I can help/clarify any further just ask.