Further to the posting below on the practice of bundling:
I said I've never bundled a bunch of partials or proposals and sent them off but in fact, when I read the comment about group rejections, I realized I have done exactly that.
Editors who work and live outside NYC come to town every couple months and set up meetings with agents. We cover a lot of ground at those meetings and frequently they'll want to see more than one project. I mail all the projects (with separate cover letters) in one envelope. Sometimes I get one rejection letter for all the projects.
It will also happen with newly hired, promoted, or just changed jobs editors. They're looking to build a list and want to see LOTS of stuff. I send it all. Again, batched stuff, and sometimes batched rejection letters.
So, yes it happens. It's such a rarity I didn't even think of it till later. The proviso is of course that every project has been requested by the editor. It's not just dropping a bunch of manuscripts in the mail room and hoping for the best.