In the first paragraph of description of the .get() function (http://api.jquery.com/get/) there is written:

If the value of index is out of bounds — less than 0 or equal to or greater than the number of elements — it returns undefined.

But, as described below on the same page and as visible in the source, when negative value is passed, the function doesn't return undefined; it returns an element with index counted from the end of the set.