The tape measure is a basic, but probably the most important tool used in clothing construction, draping and pattern making.
It's a vital tool. A wrong measurement is a wrong measurement and nothing works with a wrong measurement.
Flexible tape measures are inexpensive, but for whatever reason we tend to want to hold on to them forever. Using it over and over. Never discarding it.
The problem with this is, the more the measure is used, the more inaccurate it becomes.
They're made of plastic. They easily stretch out of shape. And most people using tape measures tend to pull on them--further distorting measurements.
To overcome this you should also use a new flexible tape measure when you're starting a new project. Or acquire a tape measure that won't stretch or wear out.
An old-fashioned metal flexible measures or paper tape measures are perfect.