In the footruler the problem is lack of accuracy and only based on waht our eyes can only see
in the vernier caliper the source of error is if you close them too hard or if you dont put them perpendicular to your axis
in the micrometer caliper the source of error is when someone hardens the thimble or you doesnt take time to accurtely put it perpendicullarly to your xis
it also requires a bit of finesse or patience