The Trypophobia subreddit is a place for posting photos of things with clusters of small holes or pits in them, like lotus plants, funguses, multi-chambered plants, and strange infections. Trypophobia is the (not medically recognized) fear of "objects with small holes." It sounded weird to me, but after clicking through the first couple-dozen links, I was massively squicked. Shown here, Ethiopian injera bread from Apple Pie, Patis, and Pâté Recipes. Normally, I love the stuff, but in the context, I have to admit, it gives me the frisson.
The most common phobia you've never heard of. (Thanks, Fipi Lele!)