For no apparent reason xfce4-xkb-plugin keeps disappearing from tray. The line in /var/log/messages may read:

xfce4-xkb-plugi[9545] general protection ip:7f46df6e9070 sp:7fffed81f668 error:0 in libc-2.9.so[7f46df66f000+140000]

xfce4-xkb-plugi[26935]: segfault at 19f000000c4 ip 00007f5f6479a070 sp 00007fff728ce718 error 4 in libc-2.9.so[7f5f64720000+140000]

Process numbers and addresses, of course, vary. 

xfce4-xkb-plugin versions 0.5.3.3 and earlier. I've seen crashes both on glibc versions 2.8 and 2.9. Distribution: Gentoo Linux.