I've noticed on the English Wikipedia that when previewing Lua modules using the "preview page with this template" option, if there is a script error, the JavaScript error message doesn't show up when you click the link. To demonstrate this, I have made the following:
https://en.wikipedia.org/wiki/Module:User:Mr._Stradivarius/previewbug
https://en.wikipedia.org/wiki/User:Mr._Stradivarius/Sandbox/Previewbug

If you click on the link in [[User:Mr. Stradivarius/Sandbox/Previewbug]] you can see the error message, but if you edit [[Module:User:Mr. Stradivarius/previewbug]] and then use "preview page with this template" with "User:Mr. Stradivarius/Sandbox/Previewbug", the error message does not appear.

However, this bug does not occur on the test2 wiki. I've made equivalent sandbox and module pages at:
https://test2.wikipedia.org/wiki/Module:User:Mr._Stradivarius/sandbox
https://test2.wikipedia.org/wiki/User:Mr._Stradivarius/sandbox

The error message displays fine with these pages. Because of this I'm not sure where the bug is. But according to [1], en.wiki and test2 are running the same version of Scribunto, so it's not a delay between patching one and patching the other. Therefore I'm submitting a bug here in case it is a software-side problem.

[1] https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_%28technical%29&oldid=547427635#Lua_error_messages_not_displaying_on_.22preview_page_with_this_template.22

Version: unspecified
Severity: normal