John Gruber wrote a hilarious rant about the single-story a in the iOS Notes app:

I absolutely despise the alternate single-story a glyph that Apple Notes uses. I use Notes every single day and this a bothers me every single day. It hurts me. It’s a childish silly look, but Notes, for me, is one of the most serious, most important apps I use. 

Since that sparked some conversation online, he followed up with a longer post about typography in early versions of the Mac system software:

…Apple actually shipped System 1.0 with a version of Geneva with a single-story a glyph — but only in the 9-point version of Geneva. At 12 points (and larger), Geneva’s a was double-story.

To me, it does make sense that 9-point Geneva would have a single-story a, since there are less pixels to draw the glyph well and to distinguish better from the lowercase e.