https://sourceforge.net/#!/projects/sshpass/files/sshpass/1.06/sshpass-1.06.tar.gz
You get an error page reading "The sourceforge.net website is temporarily in static offline mode." Hence the SHA256 mismatch, since it's downloading an HTML page instead of a tarball. This also explained why the install worked fine for me on one Mac yesterday, but failed on another today. (I'm also finding multiple other installs via Homebrew are failing today, probably all due to the SourceForge outage. SourceForge confirmed.)
I found an alternative URL that worked and updated the sshpass.rb script to point to it:
url 'https://fossies.org/linux/privat/sshpass-1.06.tar.gz'
Installed without issue after saving the updated Ruby script to my server and running the brew install
command pointing to that URL.