To reproduce, make a typo in extension.json. The following will appear in logs:
[Wed Aug 26 04:03:15.514476 2020] [php7:notice] [pid 1016] [client 127.0.0.1:40328] PHP Fatal error: Uncaught Exception: /var/www/html/trunkwiki/extensions/Something/extension.json is not a valid JSON file. in [...]
[Wed Aug 26 04:03:15.520782 2020] [php7:notice] [pid 1016] [client 127.0.0.1:40328] PHP Fatal error: Uncaught Error: Class 'WebRequest' not found in /var/www/html/trunkwiki/includes/HeaderCallback.php:63\nStack trace:\n#0 [internal function]: MediaWiki\\HeaderCallback::callback()\n#1 {main}\n thrown in /var/www/html/trunkwiki/includes/HeaderCallback.php on line 63
This is possibly harmless (because invalid extension.json is fatal error anyway), but the cause of this message is unclear, + message is strange/confusing and shouldn't be in the logs.
This didn't happen in 1.34, but happens in 1.35.