Often one can set environment variables to enable debug logging applications and libraries for help tracing problems. This page collects those (please add in alphabetical order).
Epiphany & Firefox
NSPR_LOG_FILE=/tmp/http.log NSPR_LOG_MODULES=nsHttp:5 epiphany
Evince
GConf
GCONF_DEBUG_TRACE_CLIENT=1 myapp
Gtk
http://library.gnome.org/devel/gtk/unstable/gtk-running.html
GStreamer
# all warnings, and full log for avi GST_DEBUG="*:2,avi*:5" totem # no ansi color GST_DEBUG_NO_COLOR=1 ...
VTE (gnome-terminal)
VTE_DEBUG=all