You can find a compilation of all the official errata pages for GURPS books here:
http://www.sjgames.com/errata/gurps/
If you've discovered an error and would like to submit a piece of errata, please write to
errata@sjgames.com with the information, including the edition and printing of the book.
If you're confused by a rule, or not sure if something is an error, you can always check the
GURPS FAQ or ask on the forums before sending something in.