Go

I challenge anybody to see the bug, who doesn't already know where it is. I added a few (truthful) comments to explain the code to non-Goers.

Try to find it yourself, if you're so inclined, before reading the comment section below (where somebody will surely spoil it.)

package main

import "fmt"

// Aim: to define an integer type that by default is printed in hex notation (0xab..)
type hexInt int

// Implementation of the Stringer interface, called by Println and friends when available.
func (h hexInt) String() string {
    // "%#x" is the alternate form of "%x", which uses lowercase a-f and adds 0x in front.
    return fmt.Sprintf("%#x", h)
}

func main() {
    // Create a variable of type hexInt with the value 42 and print it.
    var h hexInt = 42
    fmt.Println(h)
}