Yeah, a few often sneak through. I just don't really worry about them, to be honest. There is a request in the Ideas section of the support portal to create another "bubble" on each element of the campaign canvas -- a red one that will hold those who get "stuck" in a step. You can vote it up if you'd like.
Hmmm... Tried to link to that idea, but it didn't work. If you search Ideas for "red" it should be near the top.