"openssl sha1" broken on Mac OS X Lion, or is there a maximum file size?
This was a faulty memory chip.

I downloaded a program to test the memory, experimented with which chips it might be, and ended up removing one, which made the memtest program stop complaining.

When I execute the "openssl sha1" command now, multiple times, it produces the same digest every time. Sadly, it's the wrong digest (as compared to the one posted on the web site), so I guess I have to redownload the file.