It's happened to all of us. You get that email "Decision on Manuscript...," open it with a bit of trepidation, just to find a (hopefully) politely worded rejection from the editor. Sometimes this is justified. Other times, however, the rejection is due to the legendary "Reviewer #2," a cranky, ill-informed, hastily written rant against your paper that is not at all fair. The details can vary--they don't like your theoretical approach, don't...
