The bug

When exiting a boat on a lower half slab and packed ice boat highway with fences on the sides, there is a chance that the player will fall through the highway.  This can be deadly if it is over a lava ocean in the Nether (like mine).

How to reproduce

Build a boat highway consisting packed ice with lower half slabs between the packed ice blocks. 

I have stone fences on either side.  Not sure if this is necessary.

Stop boat about half way between the packed ice blocks. 

Exiting will either put the player on the fences or under the highway.

I will try to attach a clip showing this.  Found this bug by accident when I decided to finally label my nether highways.  Exiting a boat to change highways I fell through and landed in a very large lava ocean and died.