From the Oxford Advanced Learner's Dictionary:
The definition above said it is not obvious, but the example sentence used obvious pitfall, is it odd?
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According to the Oxford Collocations Dictionary, "pitfall" can take any of the following adjectives:
Notice that the definition says that the danger or difficulty is usually not obvious at first, and the second example presents the need to rely on professional advice to avoid pitfalls which, even if obvious to experts, may not be so evident to the layman.
The definition also says especially one that is hidden or not obvious, the base meaning is still a danger or difficulty.
Obvious pitfall sounds natural enough to me.
Even though pitfalls are not obvious by nature, among such pitfalls they could be rated as more or less obvious.
In this sense, you can have most/more/less/least obvious pitfalls.
I think there's a difference in perspective in the meaning of "obvious" in the two sentences:
a danger or difficulty, especially one that is hidden or not obvious at first
That is, not obvious to the person about to fall into it.
Getting professional advice will help you avoid the most obvious pitfalls
That is, obvious to me, the person telling you to get advice. The pitfall will not be obvious to you - that's why you should get advice, to discover what it is. (At which point, it's obvious, and hence, not a pitfall anymore. Which hopefully helps you avoid it. :))