The span-8 had a tip ejection error. The tip detectors said tips were still loaded after a tip eject, but they were not. Phil the Beckman guy was called to fix the problem.
The problem was specifically with probe 1, and it occured intermittently. Phil opened the system up and replaced the tip detection switch, but that wasn't necessarily the problem. He also opened up some board and reseated the tip detect wires. This rectified the problems and the span-8 has run without tip detect errors since then.