Hi Gerd,
if you are talking about the messages from ShapeMergeFilter: A possible reason is a self intersecting polygon. The filter assumes that shapes are not self intersecting, but it doesn't test this yet.
About a week ago, I got a message that I thought was about merging adjacent (non-overlapping) shapes that carry identical tags. I cannot find such a message now. Here would be two examples of this: http://www.openstreetmap.org/browse/relation/1225936 http://www.openstreetmap.org/browse/relation/1232121 These are hyper-precise multipolygons with two circles. Both the outer ring (actually, the relation) and the inner ring carry the same tag landuse=grass. Only the value of the golf=* attribute differs, but this does not matter to the default style. There used to be two polygons for the inner circle: the role=inner polygon with no tags, and a duplicate polygon with the tags landuse=grass,golf=green. I deleted the duplicate polygon and moved the tags to the role=inner polygon. Tomorrow or the day after, I should see if the mkgmap message is changing. There still are a few hyper-precisely mapped multipolygons, for which mkgmap is emitting messages. It is no problem for me to keep these relations in my ignore list. I usually monitor the output with "grep -vf logging.ignore mkgmap.log.0". Many relations that were on my ignore list are no longer generating errors. Either the data has been changed, or the higher precision is helping, or both. Marko