I bought a cheap door anchor off the internet for four dollars last month. It is just a loop of black nylon webbing with a piece of hard foam wrapped around a plastic core. You are supposed to shut the foam side in the doorframe and loop a heavy resistance band through the nylon side to do pulldowns. I set it up in my bathroom door last Tuesday afternoon.
I was trying to do a kneeling lat pulldown. I did not realize my bathroom door opens outward into the hallway. I pulled down on the heavy black band with both hands. The door immediately popped open because the interior latch was loose. The foam anchor shot out of the gap at maximum speed and hit me directly in the collarbone. It left a solid red mark on my skin for two days.
Now I only use the anchor on my front door. It is the only heavy, solid wood door in my entire apartment. It also opens inward. That means my body weight pulls the door directly against the wooden door jamb instead of the metal latch.
The cheap foam anchor actually works perfectly fine if you understand basic physics. I obviously did not. The nylon is starting to fray at the edges after a month of heavy pulling, but the stitching still holds. If you buy one of these cheap foam anchors, never use it on a hollow interior door. Find a solid frame and check the latch twice before you put your full weight on the band.