Removing "production error" since it's to track errors we're seeing in production (opposite of this :D) Tagging in SRE Observability in case there's something in the logging pipeline to investigate.

Hi Folks, had an internal team discussion, and we dont think there is anything actionable on our end. Please re-tag if you have any follow-ups. Thanks!

I see this error in my local environment (PHP 8.1, Apache) and am confused as to 1) why we don't see this error logged in production and 2) why this doesn't cause more serious issues related to central login, as the console error (Uncaught SyntaxError: Unexpected token '<' (at checkLoggedIn?type=script&wikiid=enwiki:2:223)) indicates that checkLoggedIn JavaScript doesn't run successfully.

In T348801#9728806, @kostajh wrote:

I see this error in my local environment (PHP 8.1, Apache) and am confused as to 1) why we don't see this error logged in production and 2) why this doesn't cause more serious issues related to central login, as the console error (Uncaught SyntaxError: Unexpected token '<' (at checkLoggedIn?type=script&wikiid=enwiki:2:223)) indicates that checkLoggedIn JavaScript doesn't run successfully.

Filed that specific error as T364116: checkLoggedIn script fails with headers already sent error

I suspect whether the error is happening at all depends on MediaWiki and/or Apache output buffering config (e.g. $wgDisableOutputCompression, mod_deflate). But T347889 was happening in production (it broke central autologin), it just did not get logged.