Third Wire in 14-3 Wire Showing 50V When There Should be None
What you are describing is called a Multi-wire Branch Circuit, and it is completely legal (given that it's done properly). 

The first problem, is that it doesn't should like the breakers feeding this circuit are handle tied together.  It should be feed by a double pole breaker, or the two breakers handles should be tied together.  You should not be able to shut off only half the circuit (red only or black only).

As for the voltage.  I'd suspect it's phantom voltage (inductive coupling), and so should not cause any problems (especially if the breakers were properly tied together).  

If the breaker for the "red" conductor is off, and current is flowing on the "black" conductor. Because the conductors are parallel, and in close proximity, a voltage may be induced on the "red" wire.  There should be no current, however.  So if a load was introduced, the voltage would drop to 0.

You can determine if it really is phantom voltage, simply by using a low impedance voltmeter.  The low impedance meter will read 0 volts, while a high impedance meter will read the phantom voltage.  An analog meter is one example of a low impedance meter, though some digital multimeters are able to operate in both modes.