Hermann Grassmann is a notable example.

Grassmann was an undistinguished student until he obtained a high mark on the examinations for admission to Prussian universities. Beginning in 1827, he studied theology at the University of Berlin, also taking classes in classical languages, philosophy, and literature. He does not appear to have taken courses in mathematics or physics.

See http://en.wikipedia.org/wiki/Hermann_Grassmann

Also "A Metaphor for Mathematics Education" written by Greg McColm (Notices, April 2007; www.ams.org/notices/200704/fea-mccolm-web.pdf) This one is not about Grassmann as such. But it gives an interesting account of why he went unnoticed by his contemporaries (including Gauss, Mobius, Cauchy, Hamilton):

Grassman’s problem may have been his lack of students and credentials (he studied philology and theology in Berlin and taught at technical schools, but never got a university post) and the novelty of his approach. And there was the opacity of his exposition.