heterozetesis

from The Century Dictionary.

• noun In logic, the fallacy of ignoration of the elench, which consists in replying to an argument different from that which the opponent has advanced, or in disproving something which the opponent has not maintained.
• noun The confusion of what one has really proved with a different proposition; the opinion that one has proved what one has not proved: a fault of logic substantially the same as an ignoratio elenchi.