In your high elf army you have all sorts of units. Select one manticore next to a moonguard and cast multitarget. This unit you have selected doesn’t cast it but other next to it/him instead. Only when you “empty” (use) all multitarget you can cast the one you actually wanted to do it, in the first time.
I have noticed the same thing with cure when you have a group of Archons, or when you have a group of white mages with a unicorn in it (in this last case it is heal group I am referring to).
I cannot say if this happens with other sort of units.
Edit: Black mage + Ice Mayden.