One of my great pet peeves with Arial is the relationship between ‘r’ and ‘n’ when set next to each other. If one compares rn and m, they’re nearly identical. The kerning is off, and the letters designed too similarly.
My mind glitches every time I see them in writing – particularly when the alternative is nonsensical or bizarrely out of place, as in the common abbreviation of postmodern as ‘pomo’, which has the unfortunate lookalike of ‘porno’ in Arial. Certainly not the sort of mistake one wishes one’s readers to make.