The “classic” timing set mistake, which I have witnessed several times, is to put the crank key straight up and line that up with the dot on the top gear.
This puts the cam advanced 2-1/2 teeth on a BBM with a double roller chain.
Most times this results in 8 bent intake valves.
But some engines actually have enough physical room to run without damage occurring.
Had a customer who was working on a 455 Olds.
Very low power, 200psi on the compression tester, no spark knock.
I told him it was likely cam timing way off.
They didn’t want to tear it down and look so they spent a summer with it being a dog while they messed around with timing, carbs, etc.
They finally pulled it apart…….and it had the “classic” mistake.
Put the timing set on correctly, way more power everywhere.
Lost a bunch of cranking pressure.