During the opening to "Fun With Flags" the British Union Jack is shown upside-down. (The wide white area in the diagonal cross should be to the viewers left)
Sheldon refers to Fermilab as a university when talking to Amy about Fermilab's plans to recommend everyone but Amy for the Nobel Prize. Fermilab is only a laboratory and not a university.
The pull shade on the door behind Penny when she's in Bernadette's office is lower at first but raised considerably in the middle of their conversation.
Howard refers to the man [Columbus] who tried to find a way to India but found "America" by mistake. Columbus did not discover the Americas. The Norse visited far earlier, and of course natives long before that via the Bering land bridge. Indeed Columbus never actually made it to what is now America.