Have you ever had someone point out a huge improbability in your plot? Small ones are okay, you can usually put them right by a sentence or two deftly inserted before the dodgy bit.
Something like, Alice mused that she had been fortunate indeed, having a father who'd insisted she take Tae Kwon Do classes to black belt standard, so that, should she ever be attacked by three men, she would win. Phew. So much for the doubter who said a girl would never have been able to defeat the baddies single-handed in Chapter Ten.
But what about the ones that wipe out your plot? In Lord of the Flies, Piggy's spectacles become crucially important as the only means of making fire. However, and isn't it lucky that no-one pointed this out to William Golding, Piggy is short-sighted. His lenses would have been concave, and useless for focusing the sun's rays.
Similar thing in the film Casablanca. The 'letters of transit' that guarantee the holder free travel anywhere in German-occupied Europe, that everyone is desperate to get their hands on? No such thing ever existed.
As for the plot holes that have been drawn to my attention in my two novels...you didn't think I was going to tell you what they were, did you?