Other Issues with Contracts

A contingency contract might only come into effect after a specified event, e.g., traveler's insurance.
In general, it is very hard to enumerate every possible thing that might go wrong.
If agent 1 does not trust agent 2 enough to pay him $1M for $500K shoes, 1 might instead give him $2 for 1 shoe, then another $2 for the next shoe, and so on. These are called leveled commitment contracts.
They allow some huge contracts to happen which would not do so otherwise because the agents are risk-averse.