Mates and resolving error messages

Hello. Working on the tutorials in the self-paced courses and I keep having problems with mates showing errors. I've searched around but I'm having difficulty figuring out why they appear and how to resolve them. Is there a listing of errors somewhere with tips about what they mean? 

As a specific example, I am working on adding a slider and get the error, "Slider 1 has error: Select faces to connect (hidden)". This did not initially appear, only after I added some other mates (not having anything to do with the slider). In fact, it seems at times everything is fine then I slide parts around then end up with an error.