Sometimes, the entire sear cage can be assembled correctly, but the upper portion of the firing pin block lifter spring can "slip" out of position, over the recessed portion of the lifter arm (i.e.: be out of place). Sometimes, using a dental pick or something like that, you can pull the upper portion of the lifter spring back into it's proper place on the side of the lifter arm. If so, and the lifter arm always returns downward after you pull the arm up and release it, it should be okay, as others have said.
CZTom, this is a good point. If Heero is seeing a deformed lifter spring it could definitely have come out of the lifter spring retaining groove on the lever. Good Call!
