Suddenly PHP plugins aren't loading functions?

This could be a plugin issue, but as I think it may be more generally a server config failure, posting here.

After making some routine adjustments to MySQL settings, I'm now having an issue with the behavior of certain plugins. For instance, registration - with anti-splog enabled - now fails. At signup, users are redirected to a blank page. Looking at our error logs, I can see why:

2010/12/16 23:53:25 [error] 28680#0: *580936 FastCGI sent in stderr: "PHP Fatal error:  Call to undefined function validate_username() in /home/noisepages/www/wp-content/plugins/buddypress/bp-core/bp-core-signup.php on line 326" while reading response header from upstream, client: 24.215.173.157, server: noisepages.com, request: "POST /makenoise/ HTTP/1.1", upstream: "fastcgi://unix:/dev/shm/php-fastcgi.sock:", host: "noisepages.com", referrer: "http://noisepages.com/makenoise/"<br />

<strike>And, in fact, tail -f /usr/local/nginx/logs/noisepages.error.log and I see we're getting tons of these errors.

Oddly, most stuff is still working fine, but I am getting some errors with fastcgi. All I changed was MySQL query caching, but aside from it not making intuitive sense that would be involved, I'm not sure if it's coincidental (we also rebooted the system).

Generally, what would cause plugins not to find their functions? I'm just trying to work through how to even troubleshoot this.

(As the error suggests, this is an nginx + PHP-FPM setup.)</strike>

Answer to my own question:

This is what happens if caching is enabled *in wp-config* for a plugin like W3TC or wp-supercache, and the plugin gets disabled. Good to know, so rather than delete, editing this. :slight_smile: