When do I get this error?

The "Nested comment" error is thrown when JSLint encounters the character sequence /* inside a multiline comment. Here's an example:

/* This is a multiline comment.
   /* It's valid JavaScript,
   but JSLint doesn't like it */

Why do I get this error?

This error is raised to highlight a potentially dangerous piece of code. While it is valid to include the /* sequence in a multiline comment, it is not valid to include */. If you were to do so, everything following the first */ would be parsed as if it is not part of the comment. Here's an example (notice the syntax highlighting which demonstrates the error):

/* This is a multiline comment.
   /* It's valid JavaScript, */
   but JSLint doesn't like it */

You can resolve this issue by simply removing /* character sequences from multiline comments:

/* This is a multiline comment.
   It's valid JavaScript,
   but JSLint doesn't like it */