In The Proton Displacement (2013), Leonard tells Arthur Jeffries (Professor Proton) that Sheldon doesn't annoy people on purpose. Just a week later, though, Sheldon appears to be intentionally annoying Leonard by forcing him to wear the itchy sweater when he knows that nothing Leonard can do will allow the completion of the task so that he will be allowed remove the sweater.
It is well established that Sheldon is incapable of lying, keeping secrets or deceiving anyone (The Loobenfeld Decay; The Irish Pub Formulation; etc...). However, in this episode he manages to keep the fact that he paid for the DVD secret for seven years.It totally goes against his character. Perhaps, it it's also in his character to get one over on someone, obey the rules, and proving himself the victor.