Its not my mistake as fortunately I've been lucky enough to not make any real big ones that weren't caught before press, but a now defunct studio I know of once had a contract for one of the grand slam tennis tournaments.

It wasn't a contract to work on any of the larger, main media campaigns they were running as this was just a small 3 or 4 person studio. So most of the work was designing extra collateral and signage as required when the need would popup in the lead up to the tournament. Obviously for a small studio, it was a super high profile client.

One of the jobs was to design pull up banners to line the players tunnel as they walked out to the court. There was one for every player in the tournament, and all of them were on display for every match, and they were just arranged in a random order.

In the first match, the two players started to walk out towards the court, and were to stop and wait at the end for the umpire before walking out. One of the players —who was one of the better players in the tournament and at the time a very well know name— was standing by his banner whilst standing at the front waiting. It was, just by chance, the second or third from the front. When he glanced at his banner, he and everyone nearby instantly picked up that his name was spelled wrong.

No more banners after that. If they were ordered differently it's likely no one would have noticed.