In .Net 4.0, when using some cursive fonts, the letter spacing asked by the fonts is not respected, resulting in overlapping characters.
I have created a simple project to reproduce the issue : https://github.com/tbroust-trepia/wpf-4-font-rendering
In itself, the problem is not that big of a deal, but when transforming this text to XPS, it results in an invalid XPS document. The problem is described here : http://stackoverflow.com/questions/23246254/xps-writer-fails-with-net-4-font-rendering-bug
We have the unfortunate restriction of needing to stay at .Net 4.0 maximum, because our product needs to run under Windows XP (it's almost 10% of our customer base).
I am perfectly open to a workaround. I currently am considering just modifying the resulting XPS, but it's a dirty fix, and it opens other potential problems.