Percy and the Big Mistake is a magazine story, illustrated using photographs from the television series.
Percy works very hard and sometimes has so much to do that he ends up running late. One day, Percy arrives late at Brendam Bay. The manager is extremely cross and says he is going to complain to the Fat Controller.
That night, Percy overhears the Fat Controller telling his driver that Percy is to go to the scrapyard tomorrow. Percy assumes that the Fat Controller is going to scrap him. Percy decides that he must prove that he is a really useful engine and run on time.
The next day, Percy is determined not to be late. As he races from job to job, he makes careless mistakes and even causes a crash with Gordon and some tar trucks. Percy is convinced that he will be scrapped now and decides to run away. When the Fat Controller hears, he sets off to find Percy.
The Fat Controller soon found Percy hiding in an old shed. Percy explains what he had heard the night before. The Fat Controller explains that he actually said Percy needs to take some scrap to the Smelters' Yard and then he can pull the mail all week as a reward for all his hard work.
So, that's exactly what Percy does and, that week, the mail isn't late even once.