Programs like the one below aren't handled properly and Ksysguard thinks they are zombies. Note that even if normal users don't create such processes that hackers will if they think it'll make it harder to for people to notice their process.

#define _POSIX_C_SOURCE 200809L

#include <assert.h>
#include <errno.h>
#include <pthread.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

struct start_args {
	pthread_t parent;
};

static void *start_routine(void *arg);

int main()
{
	int err = 0;

	struct start_args *start_args = malloc(sizeof *start_args);
	if (0 == start_args) {
		perror("malloc");
		return EXIT_FAILURE;
	}

	start_args->parent = pthread_self();

	pthread_t child;
	{
		pthread_t xx;
		err = pthread_create(&xx, 0, start_routine, start_args);
		if (err != 0) {
			errno = err;
			perror("pthread_create");
			return EXIT_FAILURE;
		}
		child = xx;
	}

	err = pthread_detach(child);
	if (err != 0) {
		errno = err;
		perror("pthread_detach");
		return EXIT_FAILURE;
	}

	pthread_exit(0);
}

static void *start_routine(void *foo)
{
	struct start_args *args = foo;

	pthread_t parent = args->parent;

	free(args);

	fprintf(stderr, "Process %i is a weirdo\n", getpid());

	int err = pthread_join(parent, 0);
	if (err != 0) {
		perror("pthread_join");
		exit(EXIT_FAILURE);
	}

	for (;;)
		pause();
}

Reproducible: Always