What it is

PHP ran out of memory while running a plugin or theme and crashed with a fatal error — a common cause of a sudden white-screen or 500 error. This isn't necessarily a bug in that plugin's code: it just needed more memory than the current limit allows.

How to fix it

Raise the PHP memory limit so the same request has room to finish, and consider deactivating the plugin that triggered it if it keeps happening. If you're not sure how, install the WordPress Plugin: its Security tab applies fixes like this one automatically.