Today, Warhammer 40,000: Rogue Trader received an update so I used Galaxy to download and save the new offline installer. When I tried to run it, it complained that a file was missing so I checked on the website and it turned out the last file of the installer (part 9) had not been downloaded by Galaxy.

I turned Galaxy off, boot it up again and try to re-download the installer with the same result: a non-functioning offline installer. In the end I just downloaded the missing file from the website and now it works.

So...now I guess, I'll just have to go check all the other offline installers I've backed up with Galaxy to make sure they are complete and working...