When even famous authors make mistakes

I recently finished reading Agatha Christie's Remember Murder (1945; title in England: Sparkling Cyanide), and, frankly, it was not worth the time I invested in it.  I'm not going to summarize the plot or do much of anything except discuss the denouement.  I think there's a spoiler here, so you might want to read carefully.

In the book, the young, vivacious wife of an older (early 40s) man dies during her birthday party, of cyanide poisoning (in her champagne); it is ruled a suicide.  Eventually, he comes to believe she was murdered, and, roughly one year after her death, stages another party, with the same guests, at the same restaurant, because he has a plan to reveal the murderer.  At one point in the evening, everyone gets up to dance; while they are waiting, a passing waiter retrieves one of the guest's purse from the floor and places it on the table.  The kicker is that he replaces it on the other side (left instead of right) of whatever dishes are on the table at her place.  Returning to the table (and arriving back at the able before anyone else), she sits so that her purse is (once again) on her right side, and everyone else sits down in the same places relative to where she is now sitting--everyone has moved one place to the left.  (I will admit that when I read the discussion of this, I was back at the Mad Hatter's Tea Party--"Clean cups!  Clean cups!  Move down!  Move down!")  (The table is obviously a round table, or someone would have had to notice.)

See, here's the thing.  No one notices.  Everyone is now sitting enough further to the left that their sight lines are different, and no one notices.  And then someone dies of cyanide poisoning (in the champagne, again).  A woman at another table, having been temporarily abandoned by her escort, has been watching the events at that table closely.  When questioned by the police, she gives a detailed account of what took place both before and after the dancing.  And she does not notice--or does not mention--the rearrangement of the seating.  This strikes me as incredible, in a literal sense.  And it is a crucial event in the book, and crucial to the unraveling of the deaths. 

And so I had to vent about it.