I find a lot of value in the "crossing buildings" JOSM validation check.
* Find a semi populated area
* Run the validations
* Check crossing buildings
The types of errors fall into one of two main categories:
- Buildings that should be air-gapped, with only a very slight intersection
- Doubled buildings
- Paths or roads through buildings
Doubled buildings are the ones I'm most interested in; they tend to occur at the edges of task manager jobs - two mappers are working independently and give two opinions on the size or shape of the building.
I tend to fix these by picking the best match; or even using the "join overlapping ways" tool and squaring the results.
I think these are the the most important of the crossed building errors, because queries on building counts can be inaccurate.
Air-gapping buildings is relatively easy - simple rotation or a slight move often works. At time, using the "un-glue" tool in JOSM is needed; and squaring afterwards.
Paths or roads often simply need slightly better accuracy to avoid routing people through living rooms.
Tools such as OSMOSE can help find places that need a bit of care and attention: