This is a very tricky question to answer because it would depend on what your definition of a plot hole is. The traditional belief is that a plot hole is something in the story that either allows for illogical things to occur or where things happen that contradicts what has already happened before. The problem with a lot of books is that authors try to be too clever and end up writing themselves into a corner. The only way out of it is to then come up with something far-fetched and illogical to resolve their conundrum. This is very disappointing for readers as they read in suspense, expecting to find out how the impossible is going to happen, only to find out that the author didn't know either and just faked it. You can think of it as the literary equivalent of those television shows that end in a cliffhanger of the hero being in a building that explodes, only to walk out at the start of the next episode looking only a little dirty. It insults the intelligence of readers and has spoiled many a good book as well. If only some authors would put a little more thought into their overall stories this wouldn't happen so much, but the problem is that a lot of authors write books in a series these days instead of stand-alone novels, which makes it way harder to keep track of all the loose threads. A good example of the latter would be A Song of Ice and Fire. Now before anyone starts attacking me, I love the books to death, but anyone who has read them will know that they are full of glaring inconsistencies. I'm not going to go into any specifics for the sake of those who have not yet read the books or watched the show, but suffice to say that they are there for all to see. The Harry Potter series is another good example, although it is quite amusing to see some of the fan theories that have popped up over the years to try and justify them!