Three things I just discovered in Priority: Thessia:
When you come into an open area and see a reaper looming directly above you, with three cannibals feasting on the corpses below you and two Asari snipers across from you, there is a ramp ahead of you and to your right which leads back and up to small balcony with two item pick-ups (there's a third item pickup below the balcony, but I found that one a long time ago).
No matter how quickly you move through that area, past the snipers and on to outpost Tykis, the reaper will be gone when the battle at Tykis begins. It simply vanishes during the pre-battle cutscene.
After leaving outpost Tykis on the final push to the temple, gunship Talon 5 is not killed by the harvesters you're fighting. No matter how many there are or how fast you kill them, a separate harvester appears, flies down from above, grapples the gunship, and drags it down. The harvesters attacking you ignore the gunship completely, and the one that destroys the gunship ignores you completely.