"Thickie" was an incredibly stupid person, but managed to come up with a simple idea for making money. He patented the Tension Sheet, which was just bubble wrap painted red with "Tension Sheet" printed on it. Users would pop the bubbles to relieve stress. The invention made him a millionaire by the age of 26.
Dave Lister used a Tension Sheet when he was upset about his lot in life. When he found a way to travel in time through photographs of the past, he gave the idea of the Tension Sheet to his younger self. When he returned to the present, Lister vanished. Rimmer later learned that it was Lister who had now made a fortune from the invention, even marrying Holden's wife. This caused Kryten and The Cat to disappear also, since Lister was never around to ensure The Cat's existence or rescue Kryten.
Rimmer decided to beat Lister at his own game. He travelled back to his old boarding school, Io House, and told his younger self about the Tension Sheet. Unfortunately for Rimmer, Fred Holden was in the next bunk and overheard the conversation. He took the idea to the patent office first, resetting the timeline almost back to normal.