I was creating a border in Photoshop, so I command-clicked the background, then went to select -> border. After that, I accidentally clicked select -> border again (when I meant to click "contract"), which resulted in three groups of marching ants, with the innermost having 45-degree corners. So I deselected and went about my business.
Ever since then, though, every font displays 45-degree corners. I've relaunched Photoshop, created a new document, checked the properties (there's no feathering), etc.—same issue.