Placing a boat next to sideways chains causes the boat to fall through the floor.

Steps to reproduce:

1. Make a 3-wide path that is at least 5 blocks long. The left and right blocks can be any solid block, while the middle block is packed ice (this is a standard setup for many nether highways).
2. Place chains parallel to the ice blocks. The chains must be one block above the rows of solid blocks on either side, and they must be placed horizontally.
3. Place down a boat. Placing the boat dead center of the packed ice will normally be fine, but placing the boat too far on the left or right side of the packed ice block will cause the boat to be placed under the chains and clip through the floor.