There are many bugs in existing fonts that users often don't even realise.
Just for a random example, consider Japanese text running down a page, going from top to bottom. Now consider an open bracket "(". You want this to actually be rotated 90 degrees and likewise for a closing bracket. Likewise also for punctuation like a japanese full stop.
This information has to be in the actual font but too many fonts do not do this correctly, leading to people filing bugs against openoffice etc.