I just started a devprev client that hadn't received the latest update.
A "bad file hash" popup appeared, but allowed me to continue. When I tried to open the current Refine Density test puzzle, a popup about .sf being an unknown file extension appeared, and the puzzle of course wouldn't open.
After I restarted, the client updated and opened normally.
Needing the most recent client in order to understand .sf files is expected.
My guess with the "bad file hash" was that there was transient communication issues with the server, which resulted in the downloaded update being corrupted. (This is exactly what the file hash is supposed to catch.) Since the downloaded file was bad, the update couldn't be installed, so you went back to the old version. Upon restarting, you were able to re-download the update and this time it wasn't corrupted.