While testing a bit further, I found another case which might potentially be dangerous:

```python

ds is the same as above, but chunksize is {"time": 1, "x": 1}

once on the coordinator

ds.to_zarr("test.zarr", compute=False, encoding={"time": {"chunks": [1]}, "x": {"chunks": [1]}})

in parallel

ds.isel(time=slice(0,1), x=slice(0,1)).to_zarr("test.zarr", mode="r+", region={"time": slice(0,1), "x": slice(0,1)}) ds.isel(time=slice(0,1), x=slice(1,2)).to_zarr("test.zarr", mode="r+", region={"time": slice(0,1), "x": slice(1,2)}) ds.isel(time=slice(0,1), x=slice(2,3)).to_zarr("test.zarr", mode="r+", region={"time": slice(0,1), "x": slice(2,3)}) ds.isel(time=slice(1,2), x=slice(0,1)).to_zarr("test.zarr", mode="r+", region={"time": slice(1,2), "x": slice(0,1)}) ds.isel(time=slice(1,2), x=slice(1,2)).to_zarr("test.zarr", mode="r+", region={"time": slice(1,2), "x": slice(1,2)}) ds.isel(time=slice(1,2), x=slice(2,3)).to_zarr("test.zarr", mode="r+", region={"time": slice(1,2), "x": slice(2,3)}) ```

This example doesn't produce any error, but the time and x coordinates are re-written multiple times without any warning. However, I don't yet know how a proper error / warning should be generated in this case. Maybe the check must be if every written variable touches all region-ed dimensions? But maybe thats overly restrictive?