Zombie Process on Wikipedia:
On Unix and Unix-like computer operating systems, a zombie process or
defunct process is a process that has completed execution but still
has an entry in the process table. This entry is still needed to allow
the parent process to read its child's exit status. The term zombie
process derives from the common definition of zombie — an undead
person. In the term's metaphor, the child process has "died" but has
not yet been "reaped". Also, unlike normal processes, the kill command
has no effect on a zombie process.
Zombies can be identified in the output from the Unix ps command by
the presence of a "Z" in the "STAT" column. Zombies that exist for
more than a short period of time typically indicate a bug in the
parent program, or just an uncommon decision to reap children.
If the parent program is no longer running, zombie processes
typically indicate a bug in the operating system. As with other leaks,
the presence of a few zombies is not worrisome in itself, but may
indicate a problem that would grow serious under heavier loads. Since
there is no memory allocated to zombie processes except for the
process table entry itself, the primary concern with many zombies is
not running out of memory, but rather running out of process ID
Don't wory, it happens sometimes. There's no need to log this kind of event.