I was just running the current Scared to Death mission and came to a room where a Lumber spawned alongside several Layoafers. I imeadiatly started after the Layoafers as the biggest threat when the Lumber just up and swated me for most of my health. While I was waddling around stuned (and surprised by the change) he did it again with no warning.
While I appreciate the increase in difficulty to one of the games easier mobs a little warning about it would have been nice.
Lumbers are quite fun, but i never noticed an instant swat. I usually go for them first, dancing around them till they swat, but with layoffers i can see the priority change.
Best of luck in your endeavors, wish i was more help