The devil and failing at what he does.

"The devil has had so little success in persecution, that if I did not
know that he and his children, according to this verse, could not but
persecute, I should think he would count it his strength to sit
still." - George Whitefield

Awesome.

p.s. the reference here is to the fall and the devil being bound to
strike at the heel of the seed.

you can read more of this sermon here:
http://hardwords.wordpress.com/2009/12/05/the-seed-of-the-woman-and-the-seed-of-the-serpent-the-promise-of-christ/
(although the full text isn't at that link).