"vfprintf %s NULL" during bulk build

While reviewing syslog to look for realpath problems I noticed these,
in case anyone cares:

GUPnPAV-1.0: vfprintf %s NULL in "URL: %s, ID: %s."
desktoptojson: vfprintf %s NULL in "Warning: %s (%s:%u, %s) "
mono: vfprintf %s NULL in "%s %2d %02d:%02d:%02d%.*s %d%s"
valac: vfprintf %s NULL in " value="%s""
xpcshell: vfprintf %s NULL in "%s: Resetting desktop app info dirs from %s to %s"
gst-plugin-scanner: vfprintf %s NULL in "%s%s"

(The last one was quite special because it was followed by "last message
repeated 44329 times", though looking at older logs that seems unusual).