My biggest error ever was so embarrassing. I worked at a tourist magazine in Virginia (of all places) doing layout and design. There is a very fancy and well-known restaurant that we did a lot of design work for and their speciality was She-Crab Soup. They hired us to create a rack card advertising their restaurant and on the back at the top, advertising as the largest item, their She-Crab Soup. So I designed a beautiful rack card and they had 100,000 of them printed to go into hotels and tourist areas around town.

We got a call from the restaurant about a week after the rack cards hit the street and come to find out I had spelled She-Crab soup as She-CRAP soup. OMG. And yes, it had gone thru proofreading. I ran a spell-check on it, but since 'crap' is a proper word, it didn't get flagged...and the proofreader missed it.

We ended up going all over town to gather up all the rack cards, then printed a sticker to cover up the 'crap' with a star bust of sorts to make it look like the soup was on special or a new item (I can't remember exactly). So I got to spend about a week putting stickers onto 100,000 or so rack cards to cover up the crappy job I had done :)  

(PS - the proofreader got fired for that and I felt REALLY terrible about it)