That’s one of the wonderful things about Shakespeare. He didn’t do anything by accident.
Good catch! I meant it to be, of course, as an example of a poem consisting purely of iambs, but, in my haste, neglected to notice how many feet per line. Therefore, I can’t think of any canonical poem in English that is written solely in iambic pentameter. If you did write such a poem, the lack of variance in the meter would be incredibly dull and boring. (Except, as in the Frost example, where its used exactly for this lulling effect.)