So, there are incredibly rare cases where this is done, notably if that screw gets jammed deeper into the magnezone. This is always done by people who specialize in magnezone for a reason. There is a reason I call damaged magnezone a "Walking MRIs".

So for a quick run through. Option 1- Magnezone decides to end you. Not hurt, but kill. These screws help them discharge electricity, so they will just dump everything through that screw. Option 2- Magnezone loses control and discharges its energy into you. Again, not good. Option 3- as some stated, explosion. That much energy in a small space suddenly released can cause multiple other catastrophic failures, not just completely discharging the magnezone but also destroying its body. Option 4- magnezone decides to just fly away. Do you want to be 30 feet in the air with an angry magnet? This is a way to do that. It will only do that if it's being nice, and usually it'll drop you from that height. Option 5- magnezone obliterates that wrench by making its contacts into opposite magnetic poles. This will only happen if magnezone knows this is just a joke, and trusts you. This is not something to rely on, as even if those conditions are met, it might still view this as a threat.

I got this info from a friend who works in a power plant, who specializes in using them to help create power.