Plot holes are probably one of the first things most people think of in terms of continuity errors. Plot holes can happen easily especially in stories written over a long period of time or in sequels. Most of these errors can be corrected by re-familiarizing yourself with the story thus far.
As an editor, I have seen plot holes take shape in several different ways. Here are a few examples:
Between the original work and the sequel, stories sometimes don’t match up. For instance, if a character is reference as dying one particular way in the original work, but when the sequel has a chance to fill in details, that account looks nothing like was originally portrayed.
Within the same work, I have seen things referenced at the beginning of the story and later on, that detail was forgotten. For instance, a character was mentioned to have been from one particular major city and was comfortable with city-life and later on in the story that character mentions that they grew up on a farm and prefer country-life with no clues leading to this revelation.
When in the process of writing, sometimes you write something that at the time seems to be important to the story. Later on, that detail that was hyped as a big deal is never mentioned again. This causes your audience confusion as to why it is even mentioned. This detail can be in the form of a character mentioned or introduced, a major clue a character uncovers, or any other instance where questions are raised for the reader ask. Note: this is different than foreshadowing to a sequel, because you are intending to use those questions to bring the readers back for more.
To prevent these moments from happening in your story, make sure to read through your previously written material every now and then. You can also make notes of where you can find those details in your work so you can reference back to it easily. In addition, ask your beta readers or anyone reviewing your book to help you catch these issues before you send it to the printer! While there may be minor things every now and then in your story, making sure that you minimize these instances is important to build your audience.