Sheldon talks about how if he had reabsorbed his sister in utero he would have a mole with hair instead of a tedious Christmas letter. Sheldon and his sister are fraternal twins and came from separate eggs, so he could not have reabsorbed her.
When we see Sheldon with his plastered nose and black eyes after Penny hits him, Howard asks 'So Sheldon, how's it feel to be beaten up by a girl?' but in 'The Killer Robot Instability', Penny punched Howard after he tried to kiss her and he ended up with the same injury.
This same type of error occurs in many episodes. When talking with Raj's cousin over the computer, his cousin looks up and to the right to talk to Raj and tell him to "Call your mother." Since the computer has a web camera at the top of the computer, the only way for the cousin to see Raj on his computer is to look straight ahead into the camera, not up and to the right like he is actually speaking with Raj in person.
In the dinner scene where they find out the ring is real Raj goes to get some tomato sauce. When he sits down his burger is slightly toppled over his chips then it cuts to Leonard and when it cuts back to Raj you can see his burger is standing straight.