Twice I have taken damage when shooting with the MP-133 in the latest (#42 and #44) stress test builds. Both times I was playing offline to see if fruit/mushroom spawning is active there (which it does not seem to be btw).
First occasion: Stress test #42 build, offline - I was out of time and had to shut down, so I was firing the weapon at various objects just for fun. When I aimed at a bush some distance away I suddenly took heavy damage, although I could survive and bandage myself.
Second occasion: Stress test #44 build, offline - I was searching Electro and inside the police station, when I opened a door a zombie policeman was standing there with its back to me. So I aimed at the back of its head, point blank range, and fired.
"You are dead."