A box cake mix is supposed to make the process of baking easier, but you can still make a mistake even with the simplest of mixes. A thin batter can lead to a cake that wrinkles or shrinks while in the oven. Removing some of the liquid or adding a few extras to the batter can thicken it up and save your dessert.
Scoop out some of the water with a spoon before stirring. This will help if you realize the mistake before mixing. The water will be the top layer. Dip a spoon into the liquid and take as much out as necessary.
Beat an additional egg into the batter. The egg will add viscosity to the mix and help it retain shape as it bakes.
Stir in one box of dry pudding mix. Adding additional dry ingredients will absorb some of the excess water and thicken the batter.