Prevent water spill from shower pipes during removal of stem
Take a 3x5 index card or similar size flexible but not to thin piece of paper and bend it into a curve so that it will slide into the opening under the stem, angle it slightly so water will run out towards you.

You may have to trim the length and width depending of the depth of the wall and the opening. It only needs to curve up slightly, you are creating kind of a half funnel, for lack of a better term. It sits under the stem so when you take stem out the water falls onto the funnel it runs out and not into the wall. 

If a few drops get past it it is no big deal, as long as you divert the majority of the water.