They won't fire you for refusing to sign something. If you are misloading often then that's probably why they asked you to sign that paper. Your sup is just doing his job, probably was told to do so by his boss. It's our responsibility to make sure you guys are putting the right packages in the right trucks, and if there is an issue we have to fix it somehow or eventually it *could* lead to termination. It's very rare loaders get fired for misloads, don't worry about that, just focus on your work and try to improve on the misloads. However, if you feel like your sup is harassing you at any point, go directly to your shop steward. Hope things get better for you