I had this idea in my campaign to rob a magic store. Logically, it makes sense that the owner of said store knows counterspell. What I plan to do is to take out the components of fireball but not cast fireball. Simultaneously, I'll take out the components for major image, concealing them, and create a "fireball", then run out with whatever stolen goods I have.
Since the spell the shop owner is countering isn't fireball, would this casting of counterspell be ineffective?
Counterspell is an "attempt to interrupt a creature in the process of casting a spell." Nowhere does counterspell say that the counter-caster needs to know what spell is being cast--in my experience they usually don't!
What does counterspell require? Well, the casting time is a reaction whose trigger is a creature within 60' casting a spell. It sounds from your description like you're a creature; I assume from the "cast and run out" description that the shop's smaller than 60', so it sounds like the trigger for the reaction is satisfied. Thus the shopkeeper can counter your major image, even if they thought is was a fireball you were intending.
(Where it seems like you might have room to head-fake the store owner like this would be if you pretended to cast fireball--perhaps some kind of Performance check?--while actually casting something lower-level in the hopes that the shopkeeper would blow their spell slot, and then drop your fireball. Of course, you really shouldn't drop a fireball in the middle of a store.)
