Errors with a liquid measuring cup usually come not from the cup but from the angle of view. When you lift the cup to eye level, it tilts and the measurement line is no longer parallel to the liquid surface. The second mistake is reading the top edge of liquid that climbs up the sides. The correct method is to place the cup on a level surface, crouch down, and read the bottom of the curve.
A liquid measuring cup is made on the assumption that its markings sit level on a horizontal surface. When the cup is level, the liquid surface is horizontal too, so comparing it with a line gives a meaningful reading. When you lift the cup by hand, two things go wrong at once: the cup is almost never held perfectly level, and your eyes look at the line from above or below. In a tilted cup, the liquid rises on one side and falls on the other; depending on which side you look at, the same amount reads differently. That is where the error comes from. This is why the accuracy of a measuring cup often depends less on its quality than on how you read it.
Your viewing angle is a separate source of error. If your eyes are above the liquid surface, the line appears to be below the liquid, so you add more; if your eyes are below it, the opposite happens. The thickness of the cup and the markings on its outer surface make this worse. The correct method is to set the cup on a level surface, such as the counter, and bring your eyes down to the line; in other words, bring your eyes to the cup, not the cup to your eyes. Crouching in front of the counter is the simplest way to measure accurately.
The third issue is that the liquid surface is not flat. Water and similar liquids cling to glass or plastic and climb slightly at the edges, forming a concave curve in the middle. This curve is called the meniscus. If you read it at the edges, the amount looks greater than it is. There is one rule for all measuring cups: read at the lowest point of the curve. The curve is less pronounced in thicker liquids such as oil, but the same rule applies. The meniscus is more visible in glass cups than in plastic ones, making reading errors easier to spot.
The shape of a measuring cup also tells you something. Liquid measuring cups are never filled all the way to the top line; there is always some room above the line. That space prevents spills while carrying the cup, and if you fill it, you will not be able to move it without spilling. The spout on the cup serves the same purpose. So if the amount a recipe calls for is close to the cup’s capacity, measuring it in two batches is both safer and more accurate than measuring it all at once.
Finally, liquid and dry measuring cups are not interchangeable. Cups made for dry ingredients are designed to be filled to the brim and leveled off with a straight knife; that is not possible with a liquid measuring cup because its lines are below the rim. When you put flour or sugar in a liquid measuring cup, you cannot level the surface, and the amount is almost always too high. For dry ingredients, the most reliable method is to use a scale instead of a cup. For the same reason, measuring spoons are filled to the brim for liquids and leveled off for dry ingredients.