A new dead key -- dead_stroke -- was added into X (https://bugs.freedesktop.org/show_bug.cgi?id=6887), used notably to input the Polish stroked l or the Vietnamese Ð. Although I can input characters with dead_stroke in plain XIM (but not in plain GTK 2.0), I can't input with the dead key when SCIM is turned on.
Step-by-step procedure to reproduce.
1) Add dead_stroke for whatever key, in one of the files in /etc/X11/xkb/symbols/
2) Compile it with xkbcomp.
3) Kill all SCIM processes, open an xterm (or a GTK application with $GTK_IM_MODULE=xim) and push [dead_stroke] + l. A Polish l appears.
4) Turn on SCIM, push [dead_stroke] + l. A plain l without stroke appears.
Expect result: A polish l (U+0142) appears with 4).