Hererra v. Collins must be considered in the (unfortunately complicated) context of post-conviction review law. There are state and federal post-conviction review procedures for dealing with new evidence. In addition, while Herrera and later cases reserve the question of whether "actual innocence" is by itself a basis for habeas relief, in practice there is a "bootstrap" Constitutional violation (usually ineffective assistance of counsel) that in conjunction with the actual innocence claim will provide a basis for habeas review. And in McQuiggin v. Perkins, the Supreme Court cleared pretty much any procedural hurdle to such claims when "actual innocence" is shown.

Here is an article that discusses the context surrounding Herrera v. Collins: http://digitalcommons.law.scu.edu/cgi/viewcontent.cgi?articl....