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Badly designed caches for example. Though I have no idea what happened in this particular case.

This (bad cache) used to happen with misconfigured shopify sites and had the same impact as described

None acknowledged publicly, but it's an open-source project with pretty open administration, so you can follow along: https://phabricator.wikimedia.org/T264370

Not scoping a variable properly (causing a variable to leak out to global scope) could also cause this.

Not usually in php, where each request has no shared state with any other (outside of apc, memcached)