Type: Conjuration
Level: 3
Casting time: 4 segments
Area of Effect: 10' radius plus 5' per caster level
Duration: Instantaneous
Components: Verbal , Material, Somatic
Description: The Sacred Cows spell immediately summons forth d4 fully grown Burmese Bulls in mid-air that are flung in the direction of the target area of effect. Reflex saves are allowed to avoid being hit by the blasting bovines.
Those struck take 10d10 damage and are knocked prone and stunned for d4 rounds but are also
Blessed (as the spell) at the same time. Additional saves against magic apply for half damage by those who are hit. The Blessing effect lasts for d6 rounds.
The caster may opt to direct each Sacred Cow to a different target within the area of effect.
Material component: an oxtail; Somatic & Verbal: hold one's index fingers up to the sides of their head, pointing upward, and incant "Mooo" three times.
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Just the first thing that came to mind when I read the thread title...moving on...
There are other systems that do "magic doesn't always work" or "magic has unusual/unexpected effects". D&D is not one of those systems. Never has been. Never should be in "RAW". Different groups/DMs who want to incorporate that "level" into the spellcasting/magic of their campaign worlds are free make up their own system to do so...or borrow/import from a system that already does.
There were, once upon a time, casting time and spell disruption rules....so, in fact, one's magic did
not "always work" as expected/might not "get off in time" before the wizard was being struck...and even the stupidest of creatures knew that when the guy in the robes starts talking funny and waving his arms...HIT HIM (if you could)!
As for the "15 minute adventuring day"...that's just something that certain gamers do/did. That falls to the DMs to either get them moving, interrupt their rest, or something to prevent it...or allow it if they didn't care. Wandering Monsters, anyone?
It was certainly NOT a "sacred cow" of D&D magic use. That's a playstyle thing...amongst "powergamers" or "min-maxers" or whatever they are called if I had to guess.
There's my two cows...er...coppers.

--SD