I have read recently some thriller which has nothing to do with a "web services" topic. I don't remember now the actual details of that thriller but I read there something which I thought might be relevant.
It was something like "If you'd like to be a good doctor then you have to continue doubt everything you have already learnt or are about to learn".
I think something similar can be advised to software engineers, myself including, and to web services developers in particular. Of course, there should be some compromise there, so that one does not become a doubts 'freak' :-) and so that the doubts don't wreck the success of the project. I think "being doubtful" is the quality which no one will ask you about at the interview :-) but having some healthy pessimism is a good thing nonetheless.