Warranty
As soon as the SAT is signed off by the end user and the contractor, a warranty period kicks off. This is usually a 12-month period (it could be longer, and is agreed upon when the contract is signed).
Usually, the warranty covers any errors in the simulation software. An example could be the simulation model not converging (process model software crash) in a certain repeated scenario.
The warranty requirements should be clearly set in the contract. In the KOM, the warranty should be agreed on by all parties. In the FDS, all stakeholders should agree as to what is a warranty item. For example, would you consider finding a mistake in the control strategy that was approved in the FAT? Usually, this is not covered by the FAT.
I always recommend that the end user buys some ongoing maintenance agreement from the OTS supplier to cover at least a site visit and 40-80 engineering hours to cover such mistakes, unless the end user has the resources to do these fixes in-house...