After running postmortems at a couple of medium to large software enterprises for a while I'd say you'd likely find that it's a quite common although not everyday thing for a bad query to impact a significant number of users if not the whole site. I've seen this happen with people hand crafting queries, using ORMs, and with a NoSQL database (mongod). Most of the ones I remember were at least partly scale related. So maybe it doesn't happen so frequently if you don't have thousands of instances or more than hundreds of thousands to millions of users.
