The Internet features among its impressively large array of fantastically fun things a “proof” that uses basic algebra to demonstrate that 2 = 1:
a = b
a2 = ab
a2 – b2 = ab – b2
(a – b)(a + b) = b(a – b) (see Difference of two squares)
a + b = b
b + b = b (remember, a = b)
2b = b
2 = 1
The flaw, of course, is that since a = b, proceeding from the fourth equation to the fifth means dividing by zero. Don’t destroy the universe plzkthx.