Hi Michal,

okay, now I was able to reproduce it, don't know what I did wrong before,
the problem occurs also with the default style, typically for motorways
in the overview map.

This is what happens:
You have two ways with equal attributes like name, type etc.
Both ways share one or more points, but they are going in opposite
directions, but they share not all points.
The LineMergeFilter doesn't care about direction and merges the two lines.
This creates a line with a spike,
Simple case:
Line1 : a->b->c
Line2:       b<-c
Result after merge:  a,b,c,b
The DouglasPeuckerFilter removes the spike between a and the last b
when the error is within the tolerance, so the remaining line is  a->b instead
of a->c

In the worst case, a long way with an almost equal long spike is "simplified"
to a much shorter way.

In older releases this situation was handled by "preserving" the merge point
in LineMergeFilter, so that the DP filter did not make this error.
I removed that feature because the "preserved" flag was stored within the
point object and was used for all higher resolutions and also for all other
lines or shapes referencing that point. The effect is that the DP filter did
not work as well as possible.
Of course I did not think about the above problem :-(

I am not yet sure how to fix this problem. I guess I should
try to detect the spike before executing the DP filter, or
find a better variant of the preserved points,
or change LineMergeFilter to avoid merging lines
when that creates a spike.
Have to get some more sleep now...

Gerd