The image doesn't fit on my SD card!
Some SD cards have errors on them and while they say "8 GB" printed on them, they may only be 7.999 GB. Not enough to notice for normal use, but a problem if you are trying to write the image.
The only solution is trying a different card. You can write an 8 GB image to a 16 GB card without problems, so even if the 16 GB card is less than 16, the 8 GB image will certainly fit.