If you forget to close the pressure valve, your pressure cooker won't reach the required pressure/cooking temperature to properly cook any food. The same goes for not putting the sealing ring into the lid. If you see a lot of steam creeping out around the lid, and the cooker isn't coming to pressure, turn it off, and carefully remove the lid to make sure the sealing ring is in position. If you have an Instant Pot, always make sure you set the valve to "sealed."