I sometimes use the "latin capital letter sharp S" character (U+1E9E). Because I can't type it on my keyboard, I use KCharSelect. This worked fine, until a while ago, when that letter was all of a sudden marked as "non-printable" and not displayed anymore.
Reporting this to and discussing with KDE upstream lead us to a behavior change in QChar::isPrint() and/or ::category(), probably introduced in Qt 4.7 (as it seems to be working correctly in earlier versions). In Qt 4.6, QChar(0x1E9E) is marked as printable and belongs to category 0 (no category); in my 4.7 build of Qt, isPrint() returns false and category is 14 (not assigned). While the capital ß certainly is a rather new addition to unicode, it's defintely assigned and has been for a few years.
I've encountered other characters with this problem before, but unfortunately didn't note down which ones.