For some of retroCPUs there exist exhaustive instruction exercisers, i.e. programs that are capable of catching the implementation errors when run on the emulator under development or on the newly made hardware like FPGA recreation.
Usually tests run some specific sequence of instructions that should result in the predefined outcome, and if there's miscomparison, the test reports fail. Zexall, which is mentioned below, runs each instruction with all the different input data and collects results by submitting them blindly to the CRC algorithm. The predefined outcome is the correct CRC, which should be the same both on real hardware and on the (probably emulated) host the test is run on.
Examples of such exercisers include i8080 exercisers like this one or the famous Z80 zexall. For DEC CPUs there exist original tests like presented here.
What other CPUs have exhaustive sets of instruction exercisers accessible today?
Of particular interest are tests for 8086 and MC68000.