Ground - which one?

Aside from our main UAV project we have recently been sidetracked with several smaller hardware projects, like our Advanced Digital Design project.
We did at least double what the spec set out and created an RS-232 based network system with error detection and a bridge board. My part was the bridge board and for some reason I could only get one of the client boards to properly communicate with it.
After several hours of pain, I realised the solution was really simple. The primary serial connection incorparated a ground connection so the boards were grounded to the same potential. The auxillary serial connection had no ground so the boards could have been at entirely different ground levels. After quickly connecting ground connections with a piece of wire everthing worked a treat.