How do you fix “Fatal error: Allowed memory size exhausted”?

That error means a PHP process ran out of memory, common during imports, large reports, or a runaway cron. We do two things: (1) trace why memory blew up (an unbounded query, an infinite loop, a leaking plugin) and fix the source, and (2) raise WP_MEMORY_LIMIT / PHP memory_limit to a safe value as a backstop. Just bumping the limit without finding the cause only delays the next crash, so we always do both.