One character error

A particular type of programming mistake that can be incredibly difficult to find and fix. Since the error consists of only a single incorrect character, it can be extremely simple for the human eye to glance over without noticing.

An example in C:

int foo = 10;
while (foo);
{
   printf ("%d", foo--);
}

$num_to_start = 5;
while ($num_to_start)
  {
    print $number_to_start++;
  }