I'm running netinst.iso from today's rawhide, and giving it a (known-good) URL
to install from on the commandline (method=http://....)

It rejects it and requests the URL again. The log says:
[$URL represents the proper URL, which I don't feel like typing out]

INFO  : URL_STAGE_MAIN - url is $URL
INFO  : trying to mount CD device /dev/sr0 on /mnt/source
INFO  : mntloop /dev/loop0 on /mnt/runtime as /mnt/source/images/stage2.img fs is 16
INFO  : transferring $URL/.discinfo to a fd
ERROR : bad HTTP response code:
INFO  : umounting loopback /mnt/runtime /dev/loop0
ERROR : LOOP_CLR_FD failed for /mnt/runtime /dev/loop0 (Device or resource busy)

I hit OK (since the URL is, in fact, correct) and it gives me the

"Retrieving                 "

message (which should probably say "Retrieving stage2.img" but that's a separate
bug). After it finishes downloading stage2.img it gives:

Error: Unable to retrieve the install image.

The log indicates that it's downloaded stage2.img but it can't LOOP_SET_FD
/tmp/stage2.img ('cuz loop0 is still in use from above).

Hitting OK takes me back to the "URL Setup" screen, and thus I'm stuck in a loop.