The magnetization of a weighed sample can influence the magnetic sensor of standard electronic balances. The corresponding weight bias is demonstrated on several meteoritic and terrestrial basalt samples previously exposed to large fields, as well as on small permanent magnets in the range 10--100 g. The mass bias is only a few mg (except for magnets which should never be brought near to an electronic balance), but it can be troublesome in case of differential weighting. Moreover, a similar magnetic bias is observed near laboratory variable field sources with significant gradient (such as electromagnets).