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In "The Monopolar Expedition" (S2E23), He sent Sheldon on a three-month National Science Foundation expedition to the Arctic Circle, and was annoyed when Sheldon visited him at his home in the middle of the night. His wife eventually set their dogs on the unwelcome visitor. Siebert does not appear on the screen.
In "The Vengeance Formulation" (S3E09), When the President visited Barry Kripke's lab to inspect a micro-controlled plasma experiment and review how a grant was used, he (along with the board of directors) became a victim of Sheldon's "foamy vengeance" on Kripke, in which large quantities of a solution of hydrogen peroxide and one of saturated potassium iodide were secured above the ceiling tiles of the lab's drop ceiling and triggered to react with ordinary dish soap, creating an exothermic release of oxygen.
Two years later, In "The Benefactor Factor" (S4E15), Siebert organized a get-together with a number of the university's wealthy donors in order to garner funds for the university's programs, and ordered Leonard, Sheldon, Howard, and Raj to attend, calling the guys his favorite geniuses. Seeing Sheldon's behavior towards the donors, however, he asks that he not appear again. He states the board of directors insists Sheldon has a beautiful mind, but Seibert thinks Dr. Cooper "is just bananas." He also encourages Leonard to have sex with a rich female donor in order to get funding.
In "The Rothman Disintegration" (S5E17), when Sheldon and Kripke were fighting over who would get Rothman's office they asked Siebert,who was urinating, to decide who gets the office he said he had to let them work it out.
In "The Parking Spot Escalation" (S6E09), when Sheldon is claiming on a parking lot spacea gainst Howard, he tries to visit Siebert's home again. and this time the President does not even answer the door. Sheldon later calls him at home, and it can be deduced from Sheldon's talking that the President is rather annoyed, and even throw "salty language" in the call. Siebert does not appear on the screen.