A couple of weeks ago, I QTed a haddoni and a gigantea in a 10 gallon tank (I ordered two gigs and the vendor accidentally sent me the haddoni). Since I'm usually home when I QT gigs, I didn't use an eggcrate divider and haven't used one in quite a while. Both of the nems were done with treatment and were well on their way to recovery, but I always keep them in QT for a week after treatment is done. I went on vacation for a few days so I placed a webcam on the QT tank so I could monitor it while I was away. One morning a woke up and checked on the tank, which was cloudy. I couldn't see either nem. The next day when I got home, I found the haddoni stuck on the powerhead and the gig falling apart. Needless to say, both nems died. From now on I will ALWAYS use an eggcrate divider. And I now believe in Murphy's Law.
The eggcrate divider that I use is wedged into the sides of the tank, but I also have another piece of eggcrate that is zip-tied perpendicular to the vertical piece to act as a base. I have additional pieces of eggcrate placed at 45-degree angles which force the divider to remain in the vertical position. So far no anemone has gotten through the barrier. When I do a water change, I remove the divider and give it a good rinse, then place it back into the tank. My fear has always been that a nem would wander and get burned by the heater; never did I think it would float and get caught on the powerhead.