Howlers
Examples exist, of mathematically correct results derived by incorrect lines of reasoning. Such an argument, however true the conclusion, is mathematically invalid and commonly is known as a howler. Consider for instance the calculation:
Although the conclusion 16/64 = 1/4 is correct, there is a fallacious, invalid cancellation in the middle step. Bogus proofs, calculations, or derivations constructed to produce a correct result in spite of incorrect logic or operations were termed howlers by Maxwell. Outside the field of mathematics the term "howler" has various meanings, generally less specific.
Read more about this topic: Mathematical Fallacy