when emerging a package with fetch restrictions on, if the file exists in distfiles but is corrupted, the message I get is the same that the one when the file is not there (something like "fetch restrictions turned on, please download manually the file at..."), emerge should say "checksum invalid" so it is easier to find why the package cannot be emerged.
I guess the file was too small, because in that case it doesn't do a checksum and tries to resume the download if possible.
This is fixed in svn r5036.
This has been released in 2.1.2_rc1-r7.
*** Bug 160016 has been marked as a duplicate of this bug. ***