As I said, this is probably an edge case but this error doesn't occur in jQuery versions prior to 1.8.

When I use the contains selector to try and target an element containing a position selector text like this:

$('li:contains("\\:first")');

I get an error:

Uncaught Error: Syntax error, unrecognized expression: :contains(" jquery-1.8.0.js:4512

Here is a demo which works fine in older versions of jQuery.