I remember a friend in graduate school throwing an early edition of Jurgen Neukirch's Algebraic Number Theory book against a wall (so hard that it split the binding) after he had worked for a number of days to reconcile something he realized was an error (or typo) in the book.
For the life of me, I cannot remember what this error was.
Has anyone come across a particularly annoying error in an early edition of Neukirch? If so, what was it?
To give context to some of the answers below, I include the original statement of this question:
What are your "favorite" annoying errors and typos in otherwise excellent sources?
Please especially try to include subtle errors that can mislead, as these are very important.
Disclaimer: The author of this quesion thinks that working to resolve an error like this is a very worthwhile exercise, at least from the standpoint of having cool war stories that make a topic memorable. I'm not sure, therefore, whether I would want to post an answer to this question myself. I posted the question because I thought it would be fun.