How to reproduce:
1. Open the "UNET_Project" project
2. Open the "Main" scene
3. Build the project
4. Enter Play mode
5. Choose "LAN Host(H)"
6. Open the built project
7. Choose "LAN Client(C)"
8. In the client press space to disconnect
9. Choose "LAN Client(C)" again
10. Press space to disconnect again, the host will throw Timeout error

Expected result: Client disconnects and no error is thrown.
Actual result: Client disconnects and Timeout error is thrown.

Reproducible with: 2017.3.0a1, 2017.2.0b4, 2017.1.0p1, 2017.1.0f3, 2017.1.0a3, 5.6.2p4, 5.6.0f1, 5.6.0a4.
Regression introduced in: 5.6.0a4.
Not reproducible with: 5.6.0a3, 5.6.0a1, 5.5.4p1, 5.5.0b5, 5.4.5p1.

Note:
with 2017.1.0a2, 2017.1.0a1, 5.6.0b1 and 5.6.0a6 client crashes when trying to connect again (gif attached).

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By Design:

The reason of this behavior is UNET doesn't have reliable disconnect. It means that client during disconnect sends disconnect packet only once, if packet dropped server will disconnect client by timeout. So, no difference between regular disconnect and timeout.