[WalterBright]

Control reversal, when the surfaces move opposite from the command, have happened. They nearly always result in a crash. How does Boeing prevent controls from being hooked up backwards?

The hydraulic actuators (rams) have an input and an output port. Connecting the hydraulic lines to the wrong port results in control reversal. To defend against that:

1. One port has left handed threads, the other right handed threads

2. The ports are different sizes

3. The ports are color coded

4. The lines cannot be bent to reach the wrong port

5. Any work on it has to be checked, tested, and signed off by another mechanic

And finally:

5. Part of the preflight checklist is to verify that the control surfaces move the right way

I haven't heard of a control reversal on airliners built this way, but I have heard of it happening in older aircraft after an overhaul.