Apparently even a difference of the least precision of the upper left-hand corner causes an alignment error in PostGIS. Consider this example, where I am comparing a land cover raster subset by an MTBS raster and the MTBS raster itself (with the goal being to add the latter to the former, creating the "burned" land cover).
```
SELECT ST_MetaData(r1.rast)
FROM
(SELECT rast
FROM geowepp_burnedarea
WHERE geowepp_burnedarea.rid = 2) r1
UNION
SELECT ST_MetaData(r2.rast)