I have constantly heard that the aliens cannot attack on their first sighting. I discovered that rule does not apply to melee enemies, apparently.
I had a Berserker show up, do his intro dance, and crush my soldier from the other side of a wall. I have had a Chryssalid show up, do his spitting intro, run up to my veteran soldier, and kill him.
I'm playing on Classic Ironman, so is this something that's specific to my difficulty? Or do melee enemies do this regardless?