I have these lines in /boot/loader.conf in order to enable a sometimes-installed serial console device:
A side effect of these lines is that entering single-user mode produces a prompt only on the external console. This was extremely confusing when attempting to boot single-user, as all messages prior to the single-user prompt are produced on the vid terminal (and on the external console), however at the moment that single-user mode is entered, the mirroring ceases. While it is now clear what happened, the effect is that single-user mode appears to be unavailable.
I have no /boot.config file that would indicate which console is meant to be used for messages.
I would suggest adding some text to the handbook page "Setting up the Serial Console"
Currently, these page makes no comment regarding single-user mode whatsoever. Informing the user that configuring the console may well have repercussions on entry to single-user mode will lessen confusion in the future.
I would therefore suggest adding the following text into section 26.6.6 "Setting up the Console from the Boot Loader", subsection 184.108.40.206 "Setting Up the Serial Console":
Note that entry to single-user mode will only honour input on one console, even if multiple consoles are indicated using the <tt>console</tt> boot option. Care must be taken that the first named console is available to the system manager entering single-user mode.
I will note that the first sentence in this section "The boot loader and the kernel to use the serial console can be specified by writing one line in /boot/loader.conf" seems very poorly written -- I do not understand what this line is attempting to say.
Additionally, there is already a section "26.6.7 Caveats"; a note here indicating possible effects on single-user boot would probably not go amiss.
How-To-Repeat: Add these lines to /boot/loader.conf, attempt to boot single user.
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