Hi,
I detected a lot of small villages in the south of Lower Saxony, Germany with missing house numbers. In some cases street names are missing as well and sometimes several houses have been merged as one building=residential. Tagging two of these villages took me about 2 hours each but there are dozens of them. I think this is quite nice work to do in case you have some lazy time or you are searching for small challenges.
I marked such villages with hint / error note (cross in red box). Activating the overlay “hints / error messages” pops them up on your screen. But I might not have detected all.
Example: https://www.openstreetmap.org/note/3056814
Some more to be found by typing in the search box:
Germany Despetal
Germany Einbeck
Thanks and have some fun!