I can't imagine tripping in Sagrada Familia, there were so many people crowded in when I went. That place is amazing though. Thinking of going again, it's been almost 7 years since I've been there and I bet it's changed some.
Definitely like Steves's books for the walks and avoiding-lines tips. Sagrada Familia is the only place where I feel crowds/lines are inevitable. The rest can be done very quickly.