See Note [Case elimination: lifted case]:

We used to do case elimination if
        (c) the scrutinee is a variable and 'x' is used strictly
But that changes
    case x of { _ -> error "bad" }
    --> error "bad"
which is very puzzling if 'x' is later bound to (error "good").
Where the order of evaluation is specified (via seq or case)
we should respect it.

c.f. Note [Empty case alternatives] in CoreSyn, which is how
I came across this.