It just means that it has decided in all it's wisdom to be lazy, and it likes to do this all the time.
True Story Bro
Case Example, the next available flag in the connectAttr and disconnectAttr commands, it will work some of the time but the rest it just shoves a middle finger in your face. It will give you a little error saying that the flag is incompatible with an attribute that has multiple inputs ex. "plusMinusAverage.input3D[0]". A quick solution for this problem is to create your own system that checks for the next available open slot. Something like this:

I ran into this problem trying to connect shaders to different shading engines and the plusMinusAverage node through Python.