I'm regularly monitoring ru-wiki category with pages with "Lua errors". I can see each day a few categories are being reported here, because they are using {{catmain}} which uses Module:TMT which load localization strings from commons structured data. Screenshot is in Russian, but it should be pretty clear what error message is. It happens on random categories usually 5-20 (of a few thousand I believe). Since the nature of the bug is random, let me know if I can do anything to help you to trace it back (e.g. additional diagnostic on the affected category?)
