I have a CentOS 6 machine that was initially installed as CentOS 6.4
in May of 2013. It’s /boot filesystem is 200M which, IIRC, was the default /boot size at the time.
The most recent kernel update (2.6.32-573.18.1.el6) fails because of lack of space in /boot. The workaround is edit /etc/yum.conf, reduce installonly_limit from 5 to something lower (I used 3), remove the oldest kernel via ‘rpm -e’, and then re-apply the update. In this case, it was necessary to use the ‘yum update’ command line vs the Update Applet due to an incomplete transaction from the failed update.