Recording and replaying computer programs includes state synchronization and thread management techniques that facilitate accurately replaying the recorded execution of a multithreaded application program. State synchronization comprises, during execution of an application program in a computer system, detecting an operation of the application program to write a memory; preventing the operation to write the memory; storing a representation of the memory; permitting the operation to write the memory, resulting in written memory; comparing the written memory to the stored representation; and in response to determining that the written memory is different than the stored representation, generating a notification that a memory state synchronization exception occurred.