- too small
- too tall (forcing an increase in line-height of a pixel or two)
The result is that some lines are too tall, and monospace layouts (like ASCII art) lose alignment.
Here is an example in three screenshots, where the "•" bullet character is rendered incorrectly. The first screenshot shows the bug on the current release. You can see that the center line takes up too much vertical space, and not enough horizontal space. This is a monospace font (apple monaco).
The second and third show the correct rendering. The second is an older emacs build I have that rendered text with Carbon. The third is Apple's native TextEdit.app, for reference.