A lot of people make the mistake of not tasting the dish while they are cooking it, waiting for it to finish cooking before trying it out instead. This may look like a small negligence, but it is actually a huge mistake and can make a huge impact on how the dish you are cooking. You may be making a mouth-watering dish, but if you do not make sure that the flavour or texture of that dish is exactly how you want it to be by regularly tasting it, you might find yourself staring at a very unappealing dish after you are done cooking and finally tasting it. Tasting the dish as you go is really important.
Instead of cooking a dish in simmering water, you end up cooking it in boiling water. A lot of people pay little attention to differentiating between simmer and boiling water. The water starts to simmer when you can see around one to two bubbles breaking at the surface every second, whereas boiling involves far more vigorous bubbling. While cooking in boiling water is faster, it leaves the dish tough, dry or cloudy. If something needs to be cooked in simmering water, make sure that is exactly what you cook it in. Anything done in a rush is bad.
Some people over-heat low-fat milk products, thinking that it is perfectly safe to do this since they can do this with cream. However, this is not the case. Milk products curdle or break if they are over-heated. In order to avoid this, cook them to a temperature of 180 degrees Celsius or less.
Learning to leave the food alone instead of turning, poking or flipping it while it is being cooked of fried is extremely important to develop a nice crust. However, people give in to their temptation and turn it too often. This results in the food not being as great at it possibly could have been.