Up to current cacti 0.8.6h, table poller_output may increase beyond reasonable size.
This is commonly due to php.ini's memory settings of 8MB default. Change this to at least 64 MB.
To check this, please run following sql from mysql cli (or phpmyadmin or the like)
select count(*) from poller_output;
If the result is huge, you may get rid of those stuff by
truncate table poller_output;
As of current SVN code for upcoming cacti 0.9, I saw measures were taken on both issues (memory size, truncating poller_output).