Tried, but could not reproduce this. Still the message shouldn't show up.

Thehelpfulonewiki wrote:

Where did you see this? Can he reproduce the error?

(In reply to comment #2)

Where did you see this? Can he reproduce the error?

It's on VPT. I've asked, no response yet.

Meanwhile, Tim Starling has said there:

It's hard to tell what the cause of this was without knowing the URL,
but it's possible that it was an exception message shown through api.php
due to $wgShowExceptionDetails being set to true. It's probably not a serious
security vulnerability, since such messages are sent with Cache-Control:
private, but again, it's hard to be sure without having details about where
it came from. We decided to turn off $wgShowExceptionDetails to be on the
safe side, the reasons it was turned on are mostly no longer relevant.

So if can't repro, I think we're fine to close fixed without prejudice to reopening...?

(In reply to comment #3)

So if can't repro, I think we're fine to close fixed without prejudice to
reopening...?

agreed

Changing to "FIXED" since there does seem a semblance of a fix here, and it's just nicer on everyone if it gets closed that way.