[bewaretheirs]

IEEE standard floating point uses a binary mantissa.

And binary has trouble representing fractions that are common in prices:

  $ bc
  obase=2
  scale=20
  1/5

1/5 in binary is a repeating binary fraction: 0.0011001100110011...

Just as you can't express 1/3 or 1/7 precisely as a non-repeating decimal fraction, you can't express 1/5 and 1/10 as a non-repeating binary fraction. As a result, most prices involving cents in currency cannot be expressed precisely as binary floating point numbers.

edit: fixed formatting