Why did spilled coca-cola on a terminal strip cause this odd voltage problem?
To elaborate on Passerby's answer, my mental model of the situation would look something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The cola caused some resistance to develop on one of the connections in the strip. When the load is not there, no current flows through the resistance so there is no voltage drop. When the load is applied, there is a voltage drop.