I recently used the popular Segoe UI Symbol font to add some glyphs to my WPF app. I ended up having to back it out because of incompatibility issues.
It turns out that in addition to the expected issues (breaking compatibility with Windows XP), Microsoft added an extra wrinkle. For Windows 8, some of the symbols were moved around inside the font, such that you can’t guarantee the same symbol will get produced by the same unicode character code. This compatibility update was released - probably to fix it, but it is not guaranteed that the end user has installed it.
Embedded fonts are of questionable legal standing, so I ended up using FontAwesome.WPF to fix the problem. I just swapped out TextBlocks that used to contain Segoe glyphs for the FontAwesome equivalent control based on TextBlock, and adjusted my style setters and templates to match. Now everything looks great!