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

a^{2} = ab

a^{2} – b^{2} = ab – b^{2}

(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.