Negative Effective Resistance
It is possible to have a loop with a mixture of positive and negative resistance, provided that the overall sum is positive or negative.  In either case, in the absence of an external current source or sink, the voltage at all points in the loop will be equal, which will in turn imply that the current must be zero.  If there were a loop where the sum of positive and negative resistances totaled zero, then it would be possible for an arbitrary amount of current to flow around the loop, creating potential differences at nodes around the loop.