1.Mod security Pattern match
tail -f /usr/local/apache/logs/audit_log
tail -f /var/log/httpd_audit_log

comment /etc/mo_security.conf
/usr/local/apache/conf/mod.......

2. script error
3. permission error
tail -f /usr/local/apache/logs/suexec_log

4. semaphore

ipcs
ipcrm -s nobodyid

or
ipcs -s | grep nobody | perl -e 'while (<STDIN>) { @a=split(/\s+/); print `ipcrm sem $a[1]`}'

or
save the script and run

#!/bin/bash

ipcs -s | grep nobody | perl -e 'while (<STDIN>) {
@a=split(/\s+/); print `ipcrm sem $a[1]`}'

killall -9 /usr/local/apache/bin/httpd
killall -9 /usr/local/apache/bin/httpd
killall -9 /usr/local/apache/bin/httpd
/scripts/restartsrv httpd

5. The error occur while taking any .pl file. For this add the line at the top of the file
#!/usr/bin/perl -w

6. repair the database which is using the page to be displayed.