How did you get on?
I thought that the rule interpretation could have been a bit tricky in places.
In no particular order, in addition to the stern light of a vessel, you could be looking at any of the following ;
Vessel under 50m at anchor - Rule 30 (b)
Power Driven Vessel under 7m max speed less than 7 kts - Rule 23(d)(ii)
Vessel under oars - Rule 25 (d)(ii)
Sailing vessel less than 7m. - Rule 25 (d)(i)
Vessel less than 12m aground - Rule 30(a)(i) (f) says that the 2 all round reds are not a requirement.
If you managed to get all those without looking them up - awesome.
Have a great day.