The fact that Silverlight in IE (as well as some other programs) doesn't allow me to input german umlauts while using my usual japanese keyboard layout... they get transformed into their non-umlaut counterparts instead (ä becomes a, ö becomes o, ß becomes s and so on)...
And always switching keyboard layouts when inputting stuff is a pain in the ass, as I usually type the umlauts via ALT+keypad anyway (I'm not even sure which umlaut is on which key) -_-