Beware of zencart debug logger!
In the recent versions of Zencart (I think starting from 1.3.9h) you will find that includes/extra_configures/enable_error_logging.php exists by default. This file will turn on the debug switch and will put the logs into your /cache folder. The problem is that it also puts lots of WARNINGS and such inside this, and ZC does generate lots of those. Within a short amount of time (depending on the number of visitors you have and number of warnings etc), this folder will be flooded with log files and will more than likely lead to many performance issues and such.
I recommend that you rename this to enable_error_logging.php.bak and only rename it backs when you want to check for an issue.