The Klingons mistook the Enterprise for a Garbage Scow and thus reasoned that it would be okay to shoot at it.
The Enterprise A didn't last long. It was 'launched' in 2286, and 'decommissioned' in 2293, with a few more holes in it than it was 'launched' with.
This ship was an oddity of the Constitution class, in that it had 80 decks, deck 1 was at the bottom and contained the brig, and deck 78 was at the top with the room that doesn't exist. It also had two deck 52s, and two deck 78s. It was the inspiration for the USS Enterprise (Alternate).