Nver noticed this before. It's not really a bug, but it causes a lot of problems.
I like to manually place cities near the coasts and rivers (or reroute the rivers near a city, it depends), and most of times when a city grows it ends up blocking the path of some water line. It happened dozens of times in my last game.
I took a picture of the last time it happened:
(http://img809.imageshack.us/img809/2706/simscr42.png)
You can see a small "no route" sign near the big city. The boat is supposed to go on the river (the red line is it's path), and I'm 100% sure that it could complete it's journey, previously. I started this game in 1820, so the line has been active for 16 years without problems. However recently the small town spawned that small bit of road that you can see in the closeup, blocking the river and all the lines that pass there.
This is a very new feature and I have rarely seen it active. But it seems it needs to be deactivated for water tiles ...
It becomes quite noticeable in Experimental, where cities are more likely to be placed near waterways. It is not a very big issue once realized what causes the "no route".
In general a very very useful new feature, thank you Dwachs! (to prissi too).
modified
That was actually built by Dwachs.
no feature - no bugs *sing* There is definitely a Bob Marley smiley missing :D
Edit: should be fixed with 4149.
I think this has triggered a new bug: I cannot raise any tiles any more.
Maybe rather the city should not terraform again as before? Or we keep this configurable.
There is an easy fix: In karte_t::can_raise in the call to can_raise_to at the third position parameter 'false' has to be inserted. I have no svn access currently.