_SERVER is gone. What could be the problem?

Question

PHP

Some kind of crap happened today with the server. 1 out of 50 requests gives _SERVER in undefined variable
I read on the Internet that the problem is with opcache. Alas, there are no accelerators on the server. I do not know in what direction to dig. Tell colleagues?
PHP 5.5.9-1ubuntu4.11 (cli) (built: Jul 2 2015 15:23:08)
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Answer 1

look for a bug in the code