Example error messages in apache log file /usr/local/apache/logs/error_log
[core:emerg] [pid 2013449:tid 140639997298560] (28)No space left on device: AH00023: Couldn't create the proxy mutex
[proxy:crit] [pid 2013449:tid 140639997298560] (28)No space left on device: AH02478: failed to create proxy mutex
In most cases this error message will not allow you to start apache, solution is to restart the server or use the command bellow.
Solution:
ipcs -s | awk -v user=nobody '$3==user {system("ipcrm -s "$2)}'