Set 5 in today's New Question game shows a major flaw in the game. There were three questions dealing with the World Trade Center tragedy all submitted by the same author. These must have been approved by an editor at about the same time
Not necessarily. I never got a satisfactory answer to this when I asked about it before but, to me, by circumstancial evidence it seems that trying to gerrymander the question queue by delaying approval of similar type/too many same category questions (which I have heard cited as the reason behind approval delay of questions) is a pointless effort because they get reinserted into the queue on a first come first served basis by submission date (technically, "last modified date" but it is effectively the same as most approvals even with small editor adjustments made, go back to their submission date. I have seen this first hand with several of my questions. Even with up to a month long wait for question approval once, when it did get approved it appeared in my "yet to be played" queue ahead of several questions which had been approved in the interim but which were submitted later.)
It seems the best enforcement measure may be as you said, to discourage authors from submittting in clusters of very similar questions. This may have just been a perfect storm of the statistically unlikely grouping of three questions from one author in a single set and the aforementioned ineffectiveness of the staggered approval method for ungrouping such a cluster.
Or it may just have been that the questions were different enough not to warrant such action at all. I too am wondering if they are all in the same category. If their only similarity is being related to the Sept 11 attacks, but they were under dfferent categories as MiraJane said it is possible three different editors would have taken them on and each would be unaware of the other two questions. I imagine the red flag system for author clumping comes in the form of looking for same category questions submitted close together by an author and manually comparing them. Setting up a filter to search for specific content seems...cumbersome at best, but more likely closer to impossible/apt to do more harm than good. For that matter such questions would be approved without fundemental complaint if coming from different authors so why would the same questions suddenly become unacceptable just for coming from a single author?
Also the questions submitted on Sept 11 2016 are being played this week, so it seems possible question submitters may have had the subject in mind and these three may not be the only ones that the editors saw at that time on it. So this overlap if questionable may have been lost in the shuffle that way.
Even if any one or several of those things happened, I would hesitate to call it a "major flaw" as even with the stars aligned so to speak to make this possible it would still be unlikely. It is more like you had a winning ticket for a pretty crappy lottery.