500 Internal Server Error: Apachesolr not Drupal

We have installed a 'solr' server for the use of our Drupal sites. It has been an interesting experience so far. Many thanks to all who have blazed the trail ahead of us.

I started getting the error

"500" Status: Internal Server Error in apachesolr_cron

on our main testing Drupal site recently. The error was showing up in  Adminstration -> Reports -> Recent log entries. I ran a few google searches and didn't find an answer. The index on this system is on the order of about 100K nodes. We had done some updates that required a 'rebuild' of the index. The error started to show up late into the rebuild process.
Well, I was looking in the wrong place. The 500 error was NOT coming from our Drupal server. It was coming from the Solr server. Something that just wasn't obvious from the error message that I could see.

Anyway, turned out that the error was very simple. The 'rebuild' of the index ran the system out of disk space. I killed off some old logs and all the RPMs that were used to install the solr system, and that provided enough space for the rebuild to finish and the old index to be removed.

All is well again, but , for all the Google searching I did on this error, and the lack of response, I thought it would be a good thing to share what fixed it for me.