Code:
        import std.stdio;
        struct S {
                short[4] x;
                this(short[4] args...) {
                        x[] = args[];
                }
                bool opEquals(in S s) const {
                        for (auto i=0; i < 4; i++) {
                                if (this.x[i] != s.x[i])
                                        return false;
                        }
                        return true;
                }
        }
        void main() {
                assert(S(1,2,3,4) == S(1,2,3,4));
        }

Output: with dmd -m64, the assertion fails. Inserting writeln's show that the elements of s in opEquals() are garbage values. Trying to print out the entire array of this or s causes a segfault.

On dmd -m32, there is no problem.

Not sure if this is related to bug 5570.