When do I get this error?

The "Expected exactly one space between '{a}' and '{b}'" error is thrown when JSLint encounters a number of spaces that is not equal to one in the following situations (there are numerous other examples that can cause this error, but these are some of the common ones):

Between any keyword (such as if, for or while) and its associated opening parenthesis:

var x = true;
if(x) {
    x = false;
}

Between the function keyword and its associated opening parenthesis, when the function is parsed as an expression (but not when it's parsed as a function declaration):

var x = function() {
    "use strict";
    return true;
};

Between a closing parenthesis and its associated opening curly brace (such as in a function statement, if statement, switch statement etc):

var x = true;
if (x){
    x = false;
}

Why do I get this error?

This error is raised to highlight a lack of convention and a deviation from a coding style. Your code will run without error if you do not change it, but may contravene best practices and look messy to other developers. If you are happy with your coding style, you can tell JSLint to ignore whitespace (or the lack of whitespace) by setting the white option to true:

/*jslint white: true */
var x = true;
if (x){
    x = false;
}

However, if you are using JSLint to enforce a consistent style, you will probably want to listen to what the error tells you, and just make sure you always use the correct whitespace:

var x = true;
if (x){
    x = false;
}