I'm getting this error on a magento installation in the server logs and the site is throwing a 500 internal server error.
I haven't changed anything that I think could cause this. Does anyone know where this error is coming from and what the fix is?
Is anything mentionned in the server logs? I am willing to bet you will find your true answer there.
500 is an internal fatal error, usually permission related, but other issues can also cause this, improperly configured vhosts, improperly configured .htaccess ... and many others. Check your server logs you should have more details on the error.
What were you trying to do that caused this error to appear? I get it from time to time when I do something labor intensive. For example, I'm in the process of importing around 10k products into my Magento build and got this same error message. In this particular instance, I'd be willing to be it has to do with either your Apache timeout setting or your PHP max_execution_time setting or a combination of both. It could also possibly be your PHP max_input_time setting if your like me and trying to upload/parse a giant CSV file full of products. Don't be discouraged either if you have to play with the numbers little bit to find the right combo to make things work. I have to try 3-4 times to get mine to run successfully.
A couple posts that helped me out were Script timed out before returning headers: php.fastcgi and http://www.magentocommerce.com/boards/viewthread/74021/#t213112.