The efficiency of explicit contracts depends on the ability of the legal system to enforce them. There is the problem of incomplete contracts from bounded rationality. It is quite often the case that relevent information is secret from one another. This is the concept of imperfect information. There also exists the problem of incomplete information.
Agents can only play the game that they think they are playing.
The problem is that one player or both does not know which game they are in. Possession of this knowledge lends bargaining power and hence a higher share of the outcome. This fact can be used to explain delays in bargaining, and why we never see immediate outcomes, as Rubinstein claims to prove. Theoretically, if you allow the time between offers to go to zero the duration of optimal conflict goes to zero. Every game cited here can be taken to be static unless otherwise labelled.