Some more information would be grand 
Sorry, I believed this was a known issue.
Some time ago there was a problem with some server, and mobs didn't deliver any damage at all. Zero. Nil. So newbies were shamelessly hunting Ulbernauts, and things like that. Obviously, immersiveness, training and economy suffered from this. Fortunately, the bug was solved and things went back to normal (see
http://hydlaa.com/smf/index.php?topic=24163.0 for info on that.)
So I was at the arena fighting some gladiators and rogues and suddenly found they weren't hurting me. Not a tiny single HP. Nothing. While just the day [IRL] before I had to use some potion now and then to stand my ground. And no, I hadn't got training or better armour in the meantime. I checked other PCs around and none of them was taking a single HP of damage. That's why I decided to post.
Who knows, maybe Talad blessed me and I suddenly became the ultimate Dermorian killing machine in Hydlaa, but somehow that didn't seem likely...
Regards
Edit- Found yet another instance of this
http://hydlaa.com/smf/index.php?topic=24815.0