People often have desires for good things. However, they can let their own desires blind them to The Truth.
It's so easy to see this, when it involves something clearly wrong, but it is more difficult when it involves, what seems to be, a righteous desire.
Feeneyism seems like one such example of this. A good desire to evangelize. Unfortunately, it seems that his desire to win souls for God, became inordinate and warped his good sense.
I'm sure there are plenty of examples of this same sort of thing, throughout human history. A father who wants so badly to support his wife and children, that he focuses on appealing to his customer's vices, in order to sell them and hence, make more money.
A wife who so desperately fears her children won't be well taken care of, decides to secretly engage in birth control or worse.
All kinds of temptations like these in life and everybody seems to be giving in, but I don't think we should "follow the crowd."