Experimental errors are errors , usually of inaccuracy, which inevitably occur in any laboratory experiments.

For instance, weights are only as good as the balances on which substances are weighed, liquid volumes depend on the accuracy of manufacture of pipettes, burettes and such, temperature measurement depends on the accuracy of the thermometer and so on. On top of this there is human reading error.

These errors are additive. The result of any experimentation should be qualified by a statement about the total estimated error eg an atomic mass calculated by experimentation could be given as 29 plus or minus 0.0002