Amps <- & -> Embedded
A scrolling text demo ran too fast in simulation, which led to a timer divider that was 25 times too small and a third simulator that found it by accident.
Using Yosys as a second simulator, independent of the Alchitry one because it reads generated Verilog, and a mutation that failed for the wrong reason.
A mutation test that failed when my reasoning said it should pass, and the simulator that reported loads writing nothing to the register file.
How to trust a CPU you wrote: tests you have seen fail, a single instruction encoder checked against the GNU assembler, and a RAM that was never there.