Signs stuck on walls under water can block pathfinding for bees. 

Repro steps

1. Create a 2x2 hole, 3 deep.

2. Put signs on the walls of the middle layer and fill the hole with water.

3. Spawn a bee in the waterfilled block under a sign.

4. Observe that often the bee will get stuck until it suffocates, it's pathfinding stuck at the sign. 

Note
I was not able to reproduce this beehaviour without filling the hole with water.