Well, twice is not really every time
but OK, I agree, a bad disk might be the prime suspect.
When you dowload disk images (or even packages) from the internet, you should check that the dowloaded file haven't got damaged in transit. The program to use is called md5sum
It's man page is pretty much self-explanatory. The site you downloaded the disk image from, should have the md5sum of the image published somewhere (usually on the same page). It's a number in hex they obtained for their image. You compute the md5 sum of the image you downloaded and compare it against their number. If the numbers match, there's a good chance that the file you dowloaded is not corrupted.
Also, it might be a bad disk burn. Or, overheating. Start by checking the image. Good luck!