Yep, see the discussion around Figure 7 in this section of the paper:
http://www.informatics.sussex.ac.uk/users/adrianth/ices96/no...They drew a graph of any units on any connected path from input to output, and then ran a search to see if any others had any effects, by clamping random units' values to 0 or 1 and seeing if that degraded the result. They found that one unconnected unit in particular degraded performance significantly (the bottom/right gray-colored one in Fig. 7): it had several active connections routed through it, but its own output went nowhere. So they hypothesized that it was modulating the signal in a way not captured by the mathematical abstraction of the FPGA as a digital circuit.