A recurring problem in the Drury Lane books---this is the most blantant example I could easily find:

In The Tragedy of Y, a clue to what happened in the murder room lies in a squeaky bed. In particular, a witness's recollections include getting up from the bed and thereby attracting the attention of the murderer. Drury Lane at this point goes over to the bed and pushes down on it to confirm that it squeaks---it was this noise that tipped off the murderer in the darkened room, and he seems satisfied by his experiment. Problem: Drury Lane is stone deaf! He should at least have asked one of the hearing people to confirm that the bed did squeak.

This sort of thing happens a great deal in these books---probably an inevitable consequence of having hearing writers and editors deal with a deaf detective. Lane is only noticeably deaf when the writers want him to be; the rest of the time he displays an amazing ability to read lips while the speaker has his face buried in his hands, is facing away, etc.

Nits for The Tragedy of Z:

Grungy nit: Internal evidence from The Tragedy of Z places it no more than twenty years after the Great War, and the date in that novel of Tuesday, September 8 forces the story to fall, 1931. The Tragedy of Y takes place the following year, and The Tragedy of Z takes place about a decade later. In that time Patience Thumm has been out of the country, largely in Europe. Why would she stay in Europe after the outbreak of WWII? And why did the characters in Drury Lane's Last Case, set not too long after Z, seem to have had no trouble moving around what should have been war-torn and/or occupied Europe? (Of course, the first two Drury Lane books were published in 1932, and the last two in 1933. . .)

At one point, Patience refers to having read The Tragedy of Y. Hello? Did EQ forget the book they had just written? I see no way, given the ending of The Tragedy of Y, that that case would have been written up for Patience's consumption---officially, it wasn't even solved.

There's a lot of dead time in this book. Often, weeks go by without anything happening. This occurs in the first two books as well, but then there are reasons (Drury Lane wants to delay in X, and there's a schedule to be kept in Y). Here, time needs to pass for trials and appeals, but the inaction of the detectives stands out as weird. Inspector and Patience Thumm are in town for a limited investigation. Most of the time, Patience has nothing to do, and even Inspector Thumm really has nothing to do after the second murder. Drury Lane, meanwhile, drops in unannounced on a priest and stays with him for at least a month or two, most of the time just spent in contemplation.

The DA behaves oddly, because the case against Aaron Dow is nonexistant but he seems to go all out against him. This is especially odd because it would be in his political interests to drag out the investigation as long as possible, so as to dig up the most dirt about the Senator's devious dealings. Instead, he fixates on Dow as soon as he hears about him. (And what's worse, the narrator invents some weird false dichotomy of evidence vs. logic, with the DA on the evidence side, even though he has no evidence against Dow!)

Unless I missed something, the case against Dow (for the first murder) consisted of:
a) he killed someone before (of course, that was manslaughter, ten years previously)
b) he apparently tried to blackmail the Senator (why this would lead to a motive for murder is never addressed)
c) the murder blow was struck with a left hand, and Dow's right hand was unusable.

Now, since the DA had (as far as I can recall) no evidence placing Dow at the scene of the crime, no witnesses who saw Dow even interact with the victim, and no motive for him to murder the Senator (a man with many enemies. . .) why did he prosecute, and why did the jury convict?

For that matter, why didn't Dow ever spill the beans about his blackmail schemes? Especially when he was sitting in the hot seat, and he had to know that he could never profit from his knowledge?

Were the authors playing with our heads? After the Warden carefully explains how he puts the prison on lockdown when there's an execution, we learn that the prison break was postponed until the day after the execution. Okay, we think, the break happened from a chain gang, and of course the Warden wouldn't allow prisoners out of the prision for chain gang duty on the day of an execution. But later we learn that that's not the reason for the delay at all! So the Warden locked down the prision and then let a chain gang out?