Everyone makes mistakes and lexicographers are no exception. Even now when most dictionaries are produced by large teams, errors ranging from typos to incorrect definitions can make their way in. After all dictionaries are big books that take a lot of work to write.
Samuel Johnson famously defined pastern as the knee of a horse (it is in fact a part of the horse’s foot) and when asked why, he is said to have replied ignorance, Madam, pure ignorance.
More recently, a famous mistake appeared in the 1934 Webster’s New International Dictionary, Second Edition, which included the word dord defined as a term in physics and chemistry meaning density. What had happened was that an editor had written on a definition slip D or d Physics & Chem. density meaning that either D (the uppercase letter) or d (the lowercase letter) could be used as an abbreviation for the word density. In the seven years it took to find and correct this mistake, it was copied into several other dictionaries.
Mistakes can also arise from omission—the editors of the First Edition of the Oxford English Dictionary decided to omit appendicitis and radium because they seemed likely to be short-lived terms!