What happens when you run the simulator "too fast"?

[SPowell42OBX]

Hi everyone, I'm designing my first homebuilt CPU using digital. Everything works perfectly if I run the main clock slowly enough. But if I run it "too fast" (around 3 or 4 khz) it gets glitchy. A simple program I have that counts upwards until it hits 255 will get part way there, and then I can tell it starts executing an opcode that isn't in the program (and isn't coded). It also definitely gets glitchier as I crank it up - at 3 khz it's sometimes ok, sometimes not. At 4 khz it fails pretty much all the time.

If I run it more slowly, 2khz or less, it works perfectly every time. Also, if I run the "speed test" it tells me the max speed is around 5 khz.

I'm just trying to confirm this is the simulation being pushed too fast, vs. some sort of subtle timing issue or something that I'll actually see in a few weeks when I start building this IRL.

Thanks for any advice and feedback!
Scott

[User32sys]

Had the same problem with my homebuild CPU. Turned out to be a glitch in the timing of the circuit....