Engineer immunity?
If the engineer is at full HP and you see an “Immune” message, it’s this trait: http://wiki.guildwars2.com/wiki/Transmute
To be clear, both will only prevent incoming condition damage. The full direct damage still goes through even though the message pops up.
Transmute has an 8% chance to work on every incoming condition, so if you hit him with, say, freeze grenade (3 vuln, 3 chill, maybe a bleed on crit) it’s not unlikely that you’ll see the immune message—even though all of your damage went through, the immune message popped up just because it blocked one of the three stacks of vulnerability.
Ahhhh ok thanks guys. I definitely did notice I wasn’t able to put conditions on him nearly as easily as normal. Might have to check out these traits :P