At first, try to download MemTest and let the computer run it few times.
In short: You had _valid_ files on disk, you downloaded _valid_ file from internet, they got combined correctly and the last check failed. Because the very same thing worked for ~70millions of other people, there must be something wrong on your side
a) It's hidden unknown bug in our code - this is highly improbable, because this very part of the code was tested multiple times in order to avoid such situations.
b) Your disk or memory is faulty - so please run the Memtest at least.
c) Totally unknown cause, probably gremlins.