In the original Fallout, you have to get the water chip and go to deal with the Master. Actually, you don't even have to do the second part if you want to join the Master's army. Considering the enormous amount of content you can skip or do in any order, it's a very big exception to what you suggested. Since it's the topic of the thread, I think it's worth noting.
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Yup, my argument doesnt hold water there.
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LOLZ puns.