It actually happens all the time. The heightened pleading standard comes from two cases - Bell Atlantic Corp. v. Twombly, 550 U.S. 544, 555 (2007) and Ashcroft v. Iqbal, 129 S. Ct. 1937, 1953 (2009). As the Twombly court described it - The "[f]actual allegations must be enough to raise a right to relief above the speculative level, on the assumption that all the allegations in the complaint are true." What this ends up requiring is that the plaintiff state specific factual allegations as to what the defendant is actually doing wrong. This sounds like a pretty good idea but in many cases is much more difficult than you'd think. Usually the difficultly arises because the information that supports your complaint, i.e. what the defendant is doing wrong, is totally within their control. You, as a plaintiff bringing an action, do not have access to this information until discovery starts, which is only after the complaint and an answer have been filed.

Think of it this way. You have a patent. You're pretty sure, but not positive, someone is infringing on it. Without the ability to inspect the supposed infringer's facilities, how do you plead with particularity factual allegations sufficient to support an infringment claim? If the infringment is blatent, sure this is easy. But in more nuanced cases (I can't think of a technical exmaple) it is possible to end up in a siuation where actual infringment is occuring but because of an inability to access the right information there is not enough available to sufficiently plead a claim.