A is for the Archanis system, officially disputed between the Federation and the Klingon Empire until the Khitomer Accords of 2293.
B is for bottomless shafts. Should never exist on a starship. Regardless of how big it is.
C is for the Ceti eel, one of the only surviving lifeforms on planet Ceti Alpha V after the planet's ecosystem was laid waste.
D is for Dark-matter nebulas.