This is Debian bug http://bugs.debian.org/658756. I'm forwarding this bug report for your information.
"I often experience what I think is a bug in the implementation
During the growth of my city, I often find cases of unauthorized
construction: I mean that some building has been constructed without
complying with regulations imposed by me.
For instance, a building is sometimes created on a, say, commercial
area, even though there are not enough adjacent commercial area cells
to accommodate for the building itself: this infringing building
overlaps with adjacent buildings or replaces road segments, thus
interrupting previously built streets.
One example of this misbehavior is included in the city I attached
to bug #657890 : the market I wanted to destroy (the one adjacent to
the hospital) was created on a commercial area cell and on an adjacent
road cell (thus interrupting the street).
I don't think this behavior is intentional: I am therefore under the
impression that it is due to an implementation bug.
It is probably due to the same bug that causes some buildings to be
created partially out of the map. This bug seems to be known by
upstream, as one screenshot  illustrates it and is labelled
"The OutOfMap panic" .
However, I don't know whether the upstream developers are aware
of the fact that the bug is still unfixed."