1) Can't copy/paste the messages, because of THAT known bug... so...
2) While SETI was down, I removed an alpha version of SETI science app that I believed was causing problems in validating, and the associated XML file.
3) When SETI came back up, BOINC Manager said "downloading setiathome_4.18_powerpc-apple-darwin" and "downloading better_banner.jpg" and "downloading 04no03etc".
4) The download of the WORK UNIT succeeded. The download of the science app went back for a retry.
5) When that work unit was completed with a computation error in 0 seconds, it downloaded another. Repeat. Appears to have downloaded about 5 WUs and immediately errored each of them out before I caught it.
6) I stopped, put an older but more-stable 'alpha' SETI science app in place, along with it's xml file and restarted. The next WU downloaded appears to be running, using the alpha science app - and it finally did get setiathome_4.18_powerpc-apple-darwin downloaded.

So... it appears that if a download of a science app fails, nothing stops BOINC Manager from trying to run WUs through that non-existent app.