I recently picked up a quest mace that has soul trap as one of it's powers. The more evil of you players probably have gotten it as well.
Since I am a mage, I gave the mace to my companion. After one fight, it was apparent that soul trap on his weapon doesn't fill the soul gems that I carry, as I predicted. So I gave him 8 petty 8 lesser, and 8 common soul gems and went hunting.
Soon after, we ran into a two random creatures on the roads. Wham-bam, they are dispatched. I focused on letting him hit each one, and I finished it off quickly to get the soul, and the sight and sound of the soul being captured appeared after each kill.
I check his inventory....8 filled common gems (lesser soul) and 8 filled lesser gems (lesser souls). So one kill filled all of the same size gems in his inventory. After 2 kills, I had 16 souls to work with.
I confirmed the bug by leaving the 8 petty gems on him, ensuring they were empty. The next petty soul creature we fought, I checked again that they were empty, let him soul trap it, and found that all 8 were filled.
At first, this may seem like a nice glitch to exploit, for those who are inclined to do so. But I disagree. It svcks. Because of it, I can't just give my companion 3 or 4 of each type of gem and get them filled over time. If you did try this, the first pack of 5 small frostbite spiders would fill all your gems up to Grand with petty souls, if you couldn't kill them before your companion got to them.
Anyone else see this glitch? I am playing on xbox.