The load cells used in almost all laboratory scales, analytical balances, and micro-  balances have, as the main element, a magnetic force motor that utilizes a very strong permanent magnet.  This magnet requires a soft iron magnetic circuit in order to retain its magnetic strength, and in order to contain the magnetic field, and thereby, produce a high intensity field in the vicinity of the force coil.  The force coil is used to generate the restoring force to balance the unknown weight.  It is the current in the force coil that is measured to determine the value of the unknown weight.  Anything that affects the strength of this magnetic field will modify the measured weight.

Anything that interacts with, or alters, the magnetic field in this load cell will have an effect on the measurement.  The case and the base of the scale are usually made of non-magnetic materials and provide no magnetic shielding.  This means that any strong external magnetic field in the area can interfere with measurements, and the user should be aware of this potential problem, especially when unexplained results occur.  The scale should not be located near the source of any strong magnetic fields that are not contained within the device that produces them.

It should be obvious at this point that trying to establish the weight of anything which has been permanently magnetized is likely to result in an erroneous reading.  In this case, the magnetic field of the object on the pan interacts with the magnetic field in the force motor circuit.  

The errors induced by the force motor’s magnetic field, when trying to weigh parts that contain soft iron, are far smaller.  If the force motor's air gap is very small, and the magnetic path is not prone to losses, the errors may be negligible.

The errors caused by magnetic interactions can be small and hard to repeat.  They can lead to great frustration until the cause is finally identified.  The best policy is probably to be aware that these problems can exist and try to avoid them by understanding the characteristics of what you are weighing.