It's not only xfce4-terminal, but also lxterminal and may be related with vte.
$ ps -el |grep Z F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 1 Z 1000 1760 1759 0 80 0 - 0 exit ? 00:00:00 lxterminal <defunct>
It's not only xfce4-terminal, but also lxterminal and may be related with vte.
$ ps -el |grep Z
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
1 Z 1000 1760 1759 0 80 0 - 0 exit ? 00:00:00 lxterminal <defunct>