Comment 17 for bug 292606

This is not actually a defect in nvidia-common. It is a defect in dkms.
The /etc/kernel/postinst.d/dkms script is sending messages to stdout.
That script is run under /usr/share/debconf/frontend, which expects that stdout will only be used for messages to it.
The actual response to the output does not occur while /etc/kernel/postinst.d/dkms is running.
The error message is deferred until the /etc/kernel/postinst.d/nvidia-common script runs.
That second script tries to send debconf requests to stdout.
That triggers the debconf error message in response to the junk that dkms wrote earlier.

Patching /etc/kernel/postinst.d/dkms to redirect stdout fixes the problem.
Adding "1>&2" to the invocation of /etc/init.d/dkms_autoinstaller keeps stdout clean.