This is the one where the author, typically an author of many many books in a given continuity, is a tad bit disorganized to the point where points of canon contradict each other.
All of these examples are from literature; this may be because a book or a series of books can be written by one author and one or no editors, while television shows and mass-produced comics tend to have a lot more creative types looking over each other's shoulders and catching such things.
- Sir Arthur Conan Doyle never seemed to be sure how many times Dr. Watson was married, whether or not he was married at any given moment, and to whom he was married. Come to think of it, Watson's first name was in question, as he's officially John but was called James by his wife in "The Man with the Twisted Lip".
- Brian Jacques' Redwall books followed a pattern where the adults in the current book were children or babies in the previous one. In The Pearls of Lutra a young mole girl named Diggum is introduced; in the next book, The Long Patrol, Diggum has grown up to be a fine young man.
- In the first book of The Elric Saga, Elric of Melniboné, we're told that Elric's mother died giving birth to him. In The Sailor on the Seas of Fate he reminisces about his childhood and his mother and father, implying that they were both alive when he was old enough to retain memories of them.