When do I get this error?

The "Missing radix parameter" error is thrown when JSLint, JSHint or ESLint encounters a call to the parseInt function that only has one argument. As of JSHint 2.3.0 the warning will only be issued if the es3 option is set to true. Here's an example:

parseInt("10");

Why do I get this error?

This error is raised to highlight a potential oversight that could lead to problems. The second argument of the parseInt function is used to specify a radix. If no radix is specified, the function can return suprising results. If the string begins with a 0 it will be interpreted as an octal (base 8) number. For example, parseInt("010") will return 8, and not 10. This behaviour was allowed by the ECMAScript 3 specification. However, here's what the ECMAScript 5 specification has to say (ES5 §15.1.2.2:

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed.

As of ECMAScript 5, this quirk of parseInt has been removed. However, since it's likely you will want your code to run successfully in older environments that do not support ES5, you should always pass a radix to parseInt:

parseInt("10", 10);

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W065. This means you can tell JSHint to not issue this warning with the /*jshint -W065 */ directive.