Floating-point errors. They cannot precisely represent all real numbers. Here is an example.

"Floating-point numbers cannot precisely represent all real numbers, and that floating-point operations cannot precisely represent true arithmetic operations, leads to many surprising situations. This is related to the finite precision with which computers generally represent numbers." (From Wiki)

Read more about my example here.
http://majedline.blogspot.ca/2012/06/floating-point-errors-they-cannot.html