Progman opened the ticket and wrote:

r9162

Its possible to build bouys and ports under bridges if you build the bridge first. To reproduce: Build any kind of bridge and place a bouy under it, the graphic of the bridge is deleted and the bouye sprite is loaded, which looks very weird. Same for building ports. This doesn't however work with channels or docks.
If you try to delete such a bridge with bouys and ports you get this assert:

openttd: /home/progman/openttd/trunk/src/bridge_map.h:124: void ClearSingleBridgeMiddle(TileIndex, Axis): Assertion `MayHaveBridgeAbove(t)' failed.

Reported version: trunk
Operating system: Linux

This issue was imported from FlySpray: https://bugs.openttd.org/task/679