It was during this very Fan Fest panel (via Comicbook) that Stephen Amell went on to confirm Slade Wilson will return in the present day of Arrow Season 6, so that means the big lie is between Diggle missing a shot and Thea being around in the present day (and speaking to Oliver, which feels like a suspicious tag-on).
A Diggle misfire seems like a pretty common thing that would happen, so did Oliver, er, Amell just confirm that Thea Queen died on Lian Yu?
If he did, that sounds like a pretty big spoiler, although still a somewhat confusing one.