What's going to happen is that the nut will round over, because there's little in this design to prevent the jaw back-driving the helix. Having used a lot of adjustable wrenches, and rounded over quite a few bolts, now I keep sets of open-end wrenches in reach on the toolbench and for adjustables I carry exclusively Knipex Pliers Wrenches, which are awesome tools. There's no way you're breaking the casting of the wrench without a big cheater bar, which I admit to having used...while wearing safety glasses.
Knipex are awesome, a good example of quality resulting in a tool that lasts much longer than the cheap tools and causing much less damage to the nuts you are working on.
