The tooltip on berserk (at least for Rex) incorrectly states that the monster will do 100% more damage. This turned out to be wrong and only increased his damage by ~50%. While it didn't make me lose... it did make me over-prepare to soak damage.
The other thing I found, in the same run, was an error with damage of the other boss. Bleaty, also in the Labyrinth, had 291 damage. I did everything I could in order to take a hit, getting to 65% physical resist, and somewhere around 148 hp.
Now, Bleaty should have done 130 damage (45% of 291=131-1 damage reduction from vine form). But, instead he did around 100~ for no reason at all. I did not have lifesteal, I only had 1 damage reduction.
I was too in-the-zone to take a screenshot of this, sadly, only when I went to report it did I realize I had forgotten. I was easily able to get a screenshot of Rex's berserk, but alas Bleaty is nowhere to be found.