Another thing to fix in/with mps: overlapping landuses/naturals
Often, mappers will overlay areas (rather than use distinct polygons or mps), which works for the map (due to rendering z-order), but not for database queries.

Examples:

• lakes within a forest (to be excluded from forest multi/polygon)
• different areas of wetland (to be marked with wetland=*; e.g. wetland with trees is wetland=swamp, not wetland on top of a regular forest)
• farmyards/residential areas/meadow embedded in farmland/forest
• industrial area embedded in residential area

Some examples can be seen nicely on the map, given that symbols are drawn after colored areas. This causes e.g. tree symbols on top of incorrectly overlaid water areas.

But: sometimes overlaps actually make sense
Examples:

• a quarry (for gravel) will often contain an artificial lake which is part of the mining operation and should therefore not be excluded from the quarry area
• a park area containing patches of grass and forest
• ornamental grass in residential areas (if too small for park)

So, as usual, judgement (and guidelines) will be required. In the above examples, the lake may or may not be part of the underlying area. That said, it's another huge area to be worked on.