I was out in the middle of the wilderness (going from Cheydinhal to Leyawiin on the east side of the river, not using the road), and I wanted to recharge my enchanted weapons a bit. Since I have Azura's Star, this is moderately straightforward, but I've noticed that sometimes soulgems with souls in them get used instead of Azura's Star (presumably a bug... I swear I have no empty soulgems, but after soultrapping I end up with an empty Azura's Star and a grand soulgem with a petty soul in it or something).
Anyway, so I put my 7 grand soulgems with grand souls in them and my 4 greater soulgems with greater souls in them onto the corpse of a mudcrab, to make sure I didn't accidentally wipe out a grand or greater soul.
You guessed it. I went soultrapping with Azura's Star for a while, but forgot to go back and pick up my soulgems

It was more than three game days before I realised, too, so the corpse would have been cleaned up/removed (even if I could find the right random mudcrab corpse... I think it was somewhere along the Panther river...)
I felt pretty stoopid.