It can happen, and you won’t know about it until the car flunks a smog check. Even then, it will be the last thing you’ll suspect since all your work was right. You may well have done the identical job on an identical car the same morning with no such effects. So what was the difference? Head work can throw off an engine’s compression ratio and valve timing enough that the car won’t run properly or can’t pass the sniff tests! In rare cases on high-performance cars, it could even allow piston-valve collisions either right away or once a bit of carbon builds up. On V-6 and V-8 engines, it can cause problems getting the intake manifold to fit and seal properly, leading to vacuum or coolant leaks. Here’s why it happens, how you can prevent it, and what to do if it’s too late.