Tools in custom tool panel disappearing

I'm using Illustrator CC 2015. I created a custom tool panel with a selection of the 10 common tools I use. I've been using this instead of the default tool panel. But as I am working I'll notice one will not be there anymore. So I will open up the default tool panel and drag the one I want back over to my custom panel. Later it will happen again, sometimes the same tool goes missing, sometimes a different one. So far it has happened with selection tools, shape tools, and the eyedropper. It's happened multiple times in the course of a few hours. It's easy to replace the tools but it's happened enough that now it's distracting a stops my progress. Any ideas?