Most likely, you are referring to a “proof” that introduces a premise that contradicts an earlier stated premise without any of the students noticing (which is a very cool trick indeed).
If it’s like the “I am god” trick, then the contradiction is using both 1=1 and 1=2 at the same time.
If it’s like the “I am god” trick, then the contradiction is using both 1=1 and 1=2 at the same time.