it happened to me too once, I failed to reproduce it though... I thought it had something to do with the campaign object data
EDIT: yep I found out what happened here.
So here's what I did: I made a custom ability in the Campaign editor (I based it on flamestrike, but I don't think it matters). Next, I copied it and pasted it in the normal object editor (in a map in the campaign!).
http://www.freeimagehost.eu/image/b3193b848666
http://www.freeimagehost.eu/image/e0890c848667
As you can see it's quite bugged though, as it has 4 times a different "Building reduction".
I think however that you rather not use this because it just contains everything you can do in a spell: summon units, target a unit and damage, and I think there will be conflicts if you have a targetting spell and an aoe spell at the same time on 1 ability. In fact I've never tried it because I'm afraid there would be conflicts. Oh and it only works in campaigns, I'm afraid.