A: I actually had to look this up (I'm a FFVII head). There are 5 in the coding of FFVII's materia as the few that they have left are just glitches and will freeze your game.

16: In Japan, this has the name "Booster". It is a combination (white)
materia.
26: In Japan, this has the name "Law". It is a command materia (yellow)
with the "Coin" ability. When you highlight the Coin option in
battle, this is shown in the info. bar: "What can I say? It's all
about the money". I guess it's a joke put in by the programmers.
3F: This magic materia (green) has the Mabarrier ability. Possibly
made obsolete due to the Barrier materia.
42: Has the name "Reflect" in the Japanese version. It cannot be used
in battle, but if it is the only materia you hold, "Magic" will
show up but you can't select it.
43: This is given the name "Rifabu" (translation: Reflect Wave) in the
Japanese version. It has the same characteristics as above.